ActionScript 3.0 :: Loading Dynamically A Big Image Into A UiLoader Component?

I am loading dynamically a big image into a uiLoader component. And using the following lines:
 
stage.addEventListener(Event.ENTER_FRAME,followFrame);    function followFrame(event:Event):void {
var dx:int = myUILoader.x + mouseX;    myUILoader.x -= dx / 10;        var dx1:int = myUILoader.y + mouseY;    myUILoader.y -= dx1 / 10;

I manage to make the image following the mouse just fine. But the image always off center, as the registration ponit of the uiLoader would be on the left size, even if I try to align it to the center of the stage by:
 
myUILoader.x = (myUILoader.stage.stageWidth / 2) - (myUILoader.width / 2);  myUILoader.y = (myUILoader.stage.stageHeight / 2) - (myUILoader.height / 2);