Smoothing is not enabled by default on standard Flex Image controls.

While this doesn’t affect the quality of images displayed at their native resolution, it does reduce the quality of scaled images.

To resolve this, I created a custom control that enables smoothing by default:

package
{
    import flash.display.Bitmap;
    import mx.controls.Image;

    public class SmoothImage extends Image
    {
        override protected function updateDisplayList
            (w : Number, h : Number) : void
        {
            super.updateDisplayList (w, h);

            // checks if the image is a bitmap
            if (content is Bitmap)
            {
                var bitmap : Bitmap = content as Bitmap;

                if (bitmap != null && bitmap.smoothing == false)
                {
                    bitmap.smoothing = true;
                }
            }
        }

    }
}


You can download this from the entry I posted to the Flex cookbook:
A better smooth Image control