How do you minimize a fullscreen application? Alt-tab? Cause I have done that plenty of times I think and never have seen something wrong with the png images (e.g. as panels).

It is good to keep that in mind though so thanks Emre, but it seems pretty safe to use png images.

Png images are nice to work with, they are not so big and you can preview them as a icon in (windows) folders, making your folder alot more clear than with lots of tga's (iirc).