Are faces and decals always png files, or is that just a coincidence?

[Xalos]

All of the faces and decals I have are .png files. What I want to know is if this is an actual requirement for decals and faces or if it just happens to be that all of the faces and decals that I have are .png files as opposed to some other image format.

[Ad Bot]

[LeetZero]

.PNG is transparent.
.GIF is transparent but not supported (I think).
Everything else is irrelevant, third party (not supported for everyone), stuffty compression and mostly non-transparent.

I personally like it being .PNG rather than an extension no one ever heard of and that would require special plug-ins and such.