ActionScript 3.0 :: Write A Class File Using A .as File Instead Of Writing Code Into The Script Tab Of A .fla File?

I am trying to write a class file using a .as file instead of writing code into the script tab of a .fla file.When I write stage.addChild(img); I get an error that stage is null. When I take out stage, and just write addChild(img); the code runs fine, but nothing appears on stage. Do I need to do something to set a stage active?
 
Is this because I don't have a Main class? My class is named after the file and I don't have a file named Main.Here is the code. As it is, it runs and I get the two trace outputs, but no graphic on my screen.
 
package  { import flash.display.DisplayObject; import flash.display.Sprite; import flash.display.Loader; import flash.events.*; import flash.net.*;  public class HelloWorldImage extends Sprite {  private var background:DisplayObject, foreground:DisplayObject;  private var url:String = "hw_jpg.jpg";  private var loader:Loader = new Loader( );    public function

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