Resizing flash poroprtionately with browser

Hi, I have a flash site that has various background images taking up the full browser window while the foreground elements remain the same. My problem is I want the background images to scale proportionately so they don’t distort. A example of what I have been trying to pull off is redtettemer.com.

This is the script I am using where portfolio_bg is the mc that has the background images:

Stage.scaleMode = “noScale”;
Stage.align = “TL”;

var stageListener:Object = new Object ();
stageListener.onResize = positionContent;
Stage.addListener(stageListener);

function positionContent():Void {
portfolio_bg._width = Stage.width;
portfolio_bg._height = Stage.height;

}

positionContent();

any ideas? thanks in advance.