I made some in the past, the one for the thumbnail is 64x64 as a file type named "thumb.png" or icon.png. You can tell in one of your decal zip files for example. Any transparency added will be colored onto the player or shaded if there's a pinch of a transparent color (good for making shades on your decal)
The decal on the player should be 256x256 and also a .png file.
What id do is make the 256x256 first, and resize to make the thumbnail for it. Then put it all in a zip file.
I'd use an image editor that has transparency (paint.net works great).
Hope this helps and I didn't miss anything; I typed this on my phone.
