Here`s a little class to build a swf file at runtime with 2 images. Just an example :

Here you could find the swfassist framework. Unfortunately its not documented very well.
Just check out the svn for some examples.

JPGEncoder click here

I made a swf file with 20 frames, each frame the shown bitmap is switched. The swf could be saved locally on your Pc.

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001
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/**
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 * Copyright 31.08.2009 Christian Sobolewski
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 * 
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 * usage : 
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 * 
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 * Parameters:
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 * 
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 * Version:
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 * 
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 * History:
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 * 
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 * SwfWrite
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 * @author Christian Sobolewski
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 */
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package {
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	import com.adobe.images.JPGEncoder;
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	import org.libspark.swfassist.io.ByteArrayOutputStream;
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	import org.libspark.swfassist.swf.io.SWFWriter;
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	import org.libspark.swfassist.swf.io.WritingContext;
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	import org.libspark.swfassist.swf.structures.FillStyle;
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	import org.libspark.swfassist.swf.structures.FillStyleTypeConstants;
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	import org.libspark.swfassist.swf.structures.SWF;
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	import org.libspark.swfassist.swf.structures.ShapeWithStyle;
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	import org.libspark.swfassist.swf.structures.StraightEdgeRecord;
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	import org.libspark.swfassist.swf.structures.StyleChangeRecord;
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	import org.libspark.swfassist.swf.tags.DefineBitsJPEG2;
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	import org.libspark.swfassist.swf.tags.DefineShape;
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	import org.libspark.swfassist.swf.tags.PlaceObject2;
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	import org.libspark.swfassist.swf.tags.RemoveObject2;
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	import org.libspark.swfassist.swf.tags.SetBackgroundColor;
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	import org.libspark.swfassist.swf.tags.ShowFrame;
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	import flash.display.BitmapData;
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	import flash.display.DisplayObject;
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	import flash.display.Sprite;
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	import flash.geom.Rectangle;
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	import flash.net.FileReference;
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	import flash.utils.ByteArray;
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	/**
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	 * @author Christian Sobolewski
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	 */
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	public class SwfWrite extends Sprite {
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		private var _swf : SWF;
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		[Embed(source="test.jpg")] 
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		public var imgCls : Class;
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		[Embed(source="logo.jpg")] 
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		public var imgCls2 : Class;
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		private var _pixels : ByteArray;
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		private var _pixelsOne : ByteArray;
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		private var _data : Array;
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		private var _maxLength : uint;
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		public function SwfWrite() {
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			_maxLength = 20;
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			var imgCls : DisplayObject = new imgCls();
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			var imgCls2 : DisplayObject = new imgCls2();
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			_swf = new SWF();
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			_swf.header.version = 9;
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			_swf.header.frameSize.xMax = 1024;
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			_swf.header.frameSize.yMax = 768;
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			_swf.header.frameRate = 2;
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			_swf.header.numFrames = 1;
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			var bgColor : SetBackgroundColor = new SetBackgroundColor();
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			bgColor.backgroundColor.fromUint(0x000066);
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			_swf.tags.addTag(bgColor);
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			var bmd : BitmapData = new BitmapData(640, 480, false);
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			bmd.draw(imgCls);
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			_pixelsOne = new JPGEncoder(50).encode(bmd);
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			var bmd2: BitmapData= new BitmapData(640, 480, false);
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			bmd2.draw(imgCls2);
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			_pixels = new JPGEncoder(50).encode(bmd2);
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			_data = [];
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			var q : Number;
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			for (var i : int = 0;i < _maxLength;i++) {
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				q = i % 2;
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				if (q == 0) {
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					_data.push(_pixelsOne);
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				} else {
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					_data.push(_pixels);
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				}
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			}
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			test();
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			var bytes : ByteArray = new ByteArray();
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			var wc : WritingContext = new WritingContext();
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			wc.length = 5000;
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			var write : SWFWriter = new SWFWriter();
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			write.writeSWF(new ByteArrayOutputStream(bytes), wc, _swf);
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			var fileRef : FileReference = new FileReference();
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			fileRef.save(bytes, "writen" + Math.random() + 100 + ".swf");
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		}
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		private function test() : void {
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			for (var i : uint = 0;i < _maxLength;++i) {
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				//				var file:File = File(files[i]);
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				var defineJPEG : DefineBitsJPEG2 = new DefineBitsJPEG2();
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				defineJPEG.characterId = (i * 2) + 1;
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				//				var fileStream:FileStream = new FileStream();
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				//				fileStream.open(file, FileMode.READ);
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				//				fileStream.readBytes(defineJPEG.jpegData);
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				//				fileStream.close();
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				//				fileStream = null;
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				defineJPEG.jpegData = _data[i];
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				_swf.tags.addTag(defineJPEG);
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				var defineShape : DefineShape = new DefineShape();
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				defineShape.shapeId = (i * 2 + 1) + 1;
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				defineShape.shapeBounds.xMax = 500;
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				defineShape.shapeBounds.yMax = 375;
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				var fillStyle : FillStyle = new FillStyle();
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				fillStyle.fillStyleType = FillStyleTypeConstants.CLIPPED_BITMAP_FILL;
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				fillStyle.bitmapId = (i * 2) + 1;
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				fillStyle.bitmapMatrix.hasScale = true;
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				fillStyle.bitmapMatrix.scaleX = 20;
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				fillStyle.bitmapMatrix.scaleY = 20;
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				var shape : ShapeWithStyle = defineShape.shapes;
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				shape.fillStyles.fillStyles.push(fillStyle);
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				var r1 : StyleChangeRecord = new StyleChangeRecord();
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				r1.fillStyle0 = 1;
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				r1.moveDeltaX = 50;
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				r1.moveDeltaY = 50;
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				r1.stateFillStyle0 = true;
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				r1.stateMoveTo = true;
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				var r2 : StraightEdgeRecord = new StraightEdgeRecord();
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				r2.verticalLine = true;
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				r2.deltaY = 375;
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				var r3 : StraightEdgeRecord = new StraightEdgeRecord();
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				r3.horizontalLine = true;
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				r3.deltaX = 500;
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				var r4 : StraightEdgeRecord = new StraightEdgeRecord();
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				r4.verticalLine = true;
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				r4.deltaY = -375;
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				var r5 : StraightEdgeRecord = new StraightEdgeRecord();
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				r5.horizontalLine = true;
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				r5.deltaX = -500;
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				shape.shapeRecords.push(r1, r2, r3, r4, r5);
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				_swf.tags.addTag(defineShape);
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				var placeObject : PlaceObject2 = new PlaceObject2();
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				placeObject.characterId = (i * 2 + 1) + 1;
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				placeObject.depth = i + 1;
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				placeObject.hasCharacter = true;
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				_swf.tags.addTag(placeObject);
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				_swf.tags.addTag(new ShowFrame());
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				_swf.header.numFrames++;
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//				for (var fadeIn:uint = 0; fadeIn < 20; ++fadeIn) {
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//					var fadeObject : PlaceObject2 = new PlaceObject2();
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//					fadeObject.depth = i + 1;
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//					fadeObject.isMove = true;
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//					fadeObject.hasColorTransform = true;
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//					fadeObject.colorTransform.hasMultiplication = true;
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//					fadeObject.colorTransform.redMultiplication = 256;
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//					fadeObject.colorTransform.greenMultiplication = 256;
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//					fadeObject.colorTransform.blueMultiplication = 256;
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//					fadeObject.colorTransform.alphaMultiplication = (((1.0 / 20) * 20) * 256);
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//					_swf.tags.addTag(fadeObject);
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//					_swf.tags.addTag(new ShowFrame());
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//					_swf.header.numFrames++;
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//				}
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//
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//				if (i > 0) {
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//					var removeObject : RemoveObject2 = new RemoveObject2();
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//					removeObject.depth = (i - 1) + 1;
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//					_swf.tags.addTag(removeObject);
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//				}
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//				for (var n:uint = 0; n < len; ++n) {
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//					_swf.tags.addTag(new ShowFrame());
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//					_swf.header.numFrames++;
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//				}
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			}
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		}
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	}
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}
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