[СлаваRa]

из приведенного мной примера (поправил на AS3):

private function imageResize(image:Bitmap, maxWidth:Number, maxHeight:Number, allowEnlarge:Boolean = false):void
{
    var ratioX:Number = maxWidth / image.width;
    var ratioY:Number = maxHeight / image.height;
    if (ratioX <= ratioY) 
    {
        if (ratioX < 1 || allowEnlarge) 
        {
            image.width = maxWidth;
            image.scaleY = image.scaleX;
        }
    } else {
        if (ratioY < 1 || allowEnlarge) 
        {
            image.height = maxHeight;
            image.scaleX = image.scaleY;
        }
    }
}