If you have ever used the Flex Image component, set its minWidth / minHeight or percentWidth / percentHeight you may have noticed some strange layout issues, and that the scaled quality is not that good. You can use 3rd party Image components to get round these issues, or this very simple load handler on the image. When using these properties, the Image component itself is NOT changed, rather the content within it. So it may appear to have scaled down, but the outer bounds of the Image component (not the bitmap within it) remain the original size.
Also, when scaling the Image component, bitmap smoothing is not enabled, and as it is private you cant do anything about it - so your scaled down image is a bit jagged. There are 3rd party Image components to get round these issues, or this very simple load handler on the image...
private function imageLoaded(event:Event):void
{
var img:Image = event.target as Image;
// re set the image source to a new Bitamp that is created from the current image
// bitmap data, but this time with smoothing enabled
img.source = new Bitmap( Bitmap(img.content).bitmapData, "auto", true );
// Set the scaling of the image to 20px
img.scaleX = img.scaleY = (20 / img.contentHeight);
}