Constraint scaling of bg image

1

Hi, i placed a fullscreen background image in my website but when i scale my browser window it also scales unproportionaly (or distorted). can anyone help me with my code so the scale is constraint to the browser window. heres the code. thanks in advance!

Stage.scaleMode = "noScale"
Stage.align = "TL";
bgTest._width = Stage.width
bgTest._height = Stage.height

myListener = new Object();
myListener.onResize = function () {
//resize background picture here
bgTest._width = Stage.width
bgTest._height = Stage.height
}
Stage.addListener(myListener);