Do we have to stick with gifs for buttons?
 

I'm customising a forum for a client and the default icons and buttons will have to go. The default setup is that all icons and buttons are gifs, which is somewhat limiting. It is far easier and better aesthetically to create buttons as 32bit pngs, especially when trying to achieve subtle and attractive transparency effects (it does away with all that nonsense of having to change background colours). The problem I'm having is that I can't find the stylesheets or templates from where the buttons are being referred to. For example, if I want to change:

forum/images/buttons/collapse_alt_collapsed.gif
for
../ collapse_alt_collapsed.png

What do I need to amend and where?

I've waded through the graphics manual and it's only concerned with gifs.

Is it do-able or are we stuck with this inferior image format?