[illuzor]

Я вот так делаю:

private function placeBackground(scaledObject:DisplayObject):void {
	scaledObject.scaleX = scaledObject.scaleY = 1;
 
	var backgroundBitmapProportion:Number = scaledObject.width / scaledObject.height;
	var stageProportion:Number = stage.stageWidth / stage.stageHeight;
 
	var scale:Number;
 
	if (backgroundBitmapProportion > stageProportion){
		scale = stage.stageHeight / scaledObject.height;
	} else {
		scale = stage.stageWidth / scaledObject.width;
	}
 
	scaledObject.scaleX = scaledObject.scaleY = scale;
	scaledObject.x = (stage.stageWidth - scaledObject.width) / 2;
	scaledObject.y = (stage.stageHeight - scaledObject.height) / 2;
}