Bitmap copy from display object

programming flash
1

i want to use bitmap copy on a image that i loaded


var walk:Loader = new Loader();
walk.load(new URLRequest("images/runner.png"));

  private function Animate(e:Event):void
    {
        var rect:Rectangle = new Rectangle(0, 0, 128, 128);
        var pt:Point = new Point(0, 0);
        frame.copyPixels(_root.walk, rect, pt);
        this.addChild(frame);
        }

Error : flash.display:BitmapData to an unrelated type flash.display:DisplayObject.

is there a work around that will remedy this?