package components
{
	import com.adobe.crypto.MD5;
	
	import flash.events.Event;
	import flash.filesystem.File;
	import flash.filesystem.FileMode;
	import flash.filesystem.FileStream;
	import flash.net.URLLoader;
	import flash.net.URLLoaderDataFormat;
	import flash.net.URLRequest;
	import spark.components.Image;
	
	public class CachedImage extends Image
	{
		
		private var _imageIdentifier:String = "";
		private var urlLoader:URLLoader = new URLLoader();
		private var _fromCache:Boolean = false;
		
		public function CachedImage()
		{
			super();
		}
		
		public function get fromCache():Boolean
		{
			return _fromCache;
		}

		public function set fromCache(value:Boolean):void
		{
			_fromCache = value;
		}

		public function get imageIdentifier():String
		{
			return _imageIdentifier;
		}

		public function set imageIdentifier(value:String):void
		{
			_imageIdentifier = value;
		}
		
		override public function get source():Object
		{
			return super.source;
		}
		
		override public function set source(value:Object):void
		{
			if(!_fromCache) {
				if(value is Class){
					// do nothing
				} else if(value is String || value is Object){
					//Load cached image
					_imageIdentifier = MD5.hash(String(value));
					
					// Cache the image
					var cacheFile:File= File.applicationStorageDirectory.resolvePath(_imageIdentifier);
					if (cacheFile.exists) {
						trace("ImageCache - Load Cached image  from : "+cacheFile.url);
						_fromCache = true;
						super.source = cacheFile.url;
						return;
					} else {
						trace("ImageCache - Cached image not found, create it");
						urlLoader.dataFormat = URLLoaderDataFormat.BINARY;
						urlLoader.load(new URLRequest(String(value)));
						urlLoader.addEventListener(Event.COMPLETE,imageLoaded);
						return;
					}
				}
			}
			trace("ImageCache - Load image from : "+value);
			super.source = value;
		}
		
		// when remote image is loaded, cache it !
		protected function imageLoaded(event:Event):void {
			var tilefile:File = File.applicationStorageDirectory.resolvePath(_imageIdentifier);
			if (tilefile.exists) {
				trace("ImageCache - Already exist, recreate it");
				tilefile.deleteFile();
			}
			trace("ImageCache - Cache image");
			var fileStream:FileStream = new FileStream();
			fileStream.open(tilefile, FileMode.WRITE);
			fileStream.writeBytes(urlLoader.data);
			fileStream.close();
			_fromCache = true;
			super.source = tilefile.url;
		}
	}
}

Retrieve remote image from an URL and store it locally.
Use this class for managing offline mode.

AS3 - Cached image from URL

script, cache

ActionScript 3