Centering and resizing

Good day
I have followed this AS3 tutorial at

http://www.kirupa.com/developer/as3/resizing_centering_as3.htm

and its very good contents is centering as nomal but i have one request how can i stop my background image from stretching when window is resized the below code am using is the same from tutorial
many many thanks.

function init()
{
stage.align=StageAlign.TOP_LEFT;
stage.scaleMode=StageScaleMode.NO_SCALE;
stage.addEventListener(Event.RESIZE, updateSize);
stage.dispatchEvent(new Event(Event.RESIZE));
krau.x = 0;
krau.y = 0;
}
init();
function updateSize(e:Event)
{
//Set background’s size
krau.width = stage.stageWidth;
krau.height = stage.stageHeight;
//center content
emage.x = stage.stageWidth/2 - emage.width/2;
emage.y = stage.stageHeight/2 - emage.height/2;
}