[cleptoman]

либо расширяем Loader и добавляем свойство bitmapName например
либо Dictionary

package {
 
	import flash.display.*;
	import flash.events.*;
	import flash.net.*;
	import flash.utils.*;
 
	public class Test extends Sprite{
 
		private var dictionary:Dictionary;
 
		public function Test(){
 
			dictionary						= new Dictionary();
			var loader1:Loader				= new Loader();
			var loader2:Loader				= new Loader();
 
			dictionary[loader1]				= "someBitmapName1";
			dictionary[loader2]				= "someBitmapName2";
 
			loader1.contentLoaderInfo.addEventListener(Event.COMPLETE,completeHandler);
			loader1.load(new URLRequest("images/1.jpg"));
			loader2.contentLoaderInfo.addEventListener(Event.COMPLETE,completeHandler);
			loader2.load(new URLRequest("images/2.jpg"));
		}
		private function completeHandler(e:Event){
			var loader:Loader			= (e.target as LoaderInfo).loader as Loader;
			loader.contentLoaderInfo.removeEventListener(Event.COMPLETE,completeHandler);
			trace(dictionary[loader]);
			loader						= null;
		}
	}
}