Did you know that you can specify a few additional parameters for the Embed tag, if you embed png images with the mxmlc?
It supports compression, quality and smoothing.

1

<s:BitmapImage source="@Embed( source='background.png', compression='true', quality='60', smoothing='false' )" />

You can find more info about this here: http://www.slideshare.net/bdeitte/mxmlc-fitter-happier-more-productive, slide #13