The bAS3 Class

package com.bdement.imagecache
{
import flash.events.Event;
 
import mx.controls.Image;
 
public class CachedImage extends Image
{
	private function onComplete(event:Event) : void
	{		
		ImageCache.add(this);
	} 
 
	public override function set source(source:Object) : void
	{
		if(!ImageCache.contains(source)) {
			addEventListener(Event.COMPLETE, onComplete); 
		}			
		super.source = ImageCache.get(source);
	}		
}
}

package com.bdement.imagecache
{
import flash.display.Bitmap;
import flash.display.BitmapData;
import flash.utils.Dictionary;
 
/**
 * 
 * @author bdement
 * 
 * This class reduces the amount of HTTP requests your application has to make
 * by storing the BitMapData of CachedImage objects in memory for reuse
 * by images created after the first one completes.
 */
public class ImageCache
{
	/**
	 * The internal representation of the cache
	 */		
	private static var cache:Dictionary = new Dictionary(false);				
	/**
	 * All the methods of ImageCache are static, so the constructor shouldn't be called.
	 * 
	 */				
	public function ImageCache()
	{
		throw new Error("You don't need to instantiate the ImageCache!");
	}	
	/**
	 * 
	 * @param imageSource The source of an image
	 * @return Whether the cache already contains the image indicated by imageSource 
	 * 
	 */				
	public static function contains(imageSource:Object) : Boolean
	{
		return cache[imageSource] != null;
	}		
	/**
	 * Retrieves the image 
	 * 
	 * @param imageSource The source of an image
	 * @return Either: The image if it exists in the cache, OR the default
	 * source that was passed in via imageSource.
	 * 
	 */		
	public static function get(imageSource:Object) : Object
	{
		if(contains(imageSource)) {
			trace("using '" + imageSource + "' from cache!");
			return new Bitmap(cache[imageSource] as BitmapData);	
		}
		return imageSource; // String or Object
	}
	/**
	 * Adds an image to the cache
	 * @param image The image to be cached 
	 * 
	 */		
	public static function add(image:CachedImage) : void
	{			
	    if (!contains(image.source))
	    {
	    	trace("adding '" + image.source + "' to cache!");
	        var bitmapData:BitmapData = 
	        	        new BitmapData(	image.content.width, 
	        	                                        image.content.height,
                                                                true,
	        		                                0x00000000);
 
	        bitmapData.draw (image.content);
 
	        cache[image.source] = bitmapData;
	    }
	}
 
}
}

<?xml version="1.0" encoding="utf-8"?>
<mx:Application 
	xmlns:mx="http://www.adobe.com/2006/mxml" 
	xmlns="*"
	width="500" height="200"
	layout="horizontal"	
 >  
	<CachedImage id="image1" source="http://www.brandondement.com/blog/wp-content/uploads/2009/08/rooster.gif" />			
 
	<mx:VBox>
		<mx:Label text="Set image2.source to..." />
		<mx:Button click="{image2.source = image1.source}" label="image1.source" toolTip="image1.source" />
		<mx:Button click="{image2.source = 'http://www.brandondement.com/blog/wp-content/uploads/2009/08/rooster.gif'}"	label="HTTP URL" toolTip="http://www.brandondement.com/blog/wp-content/uploads/2009/08/rooster.gif" />
		<mx:Button click="{image2.source = null}"	label="null" />
	</mx:VBox>
 
	<CachedImage id="image2" />
 
</mx:Application>