This simple style adds a background to .png and .gif images that is only shown with transparent images, so you can easily see if they are transparent or not.
I made it first of all for me, so don't tell me you don't like it. If you like it, use it, and edit as you want.

Original images for screenshots are there:
http://ftp.gnome.org/pub/gnome/teams/art.gnome.org/archive/backgrounds/gnome_plosion.png
http://mgeisler.net/images/alpha.png

Known issue:
- this does not affect data:URI images, as they can't have a filename...