.PNG are "Macromedia Fireworks" files, or any program I assume by Macromedia, forget their more common name at the moment.
PNG saves stuff in layers, aka vectors, and such. (Vectors are really cool, you could zoom up to it 1 x10^123214412414124124 and it wont get pixelated.)
You can download a month-free trial of it on their site, but ironically, Fireworks doesn't export as .tga's for some reason, so I use Ifranview to convert it to the .tga from a .jpg. The tool is somewhat similar to photoshop, but alot more "beginner friendly" and not nearly as advanced, but it is good for editing images and such, but I am yet to find a Alpha Channel section in fireworks
You can of course convert the vectors into 2-D dimensions, and .jpg's and such, but you can no longer use the cool things you can with the vectors, like bevels and such
Sorry for the long post :emote_neutral:
Edit: If you want a link to the site, PM me, as I don't want to make my post sound like a total advertisment, unless an admin gives me the okay, then ill post it somewhere I guess.