I've run into a situation where the exact same PNG file will show up as transparent only when it's not embedded. Here is the mxml:
<mx:Image source="@Embed('assets/user-big.png')"/> <mx:Image source="assets/user-big.png"/> Including both images on the page, the first is missing the transparent background. The second is fine. I have manually checked the images, both in the dev directory and bin directory. In fact, I have manually copied the dev ones over the top of the bin ones to make sure. If I drag the images to Firefox, both appear with no background. The background color is teal, so I would definitely notice. Near as I can tell, the embed is causing the background transparency to not be processed. I can't find anything in the docs saying this is expected behavior. -- Jason