Board Thread:Support Requests - Designing Your Wikia/@comment-25990980-20151118233628/@comment-4142476-20151119165425

I also think it must be a .jpg file, so regardless you're required to save it as .jpg (and be below 300kB).

@Edit: you can do this with GIMP if you have that, but with a quick google you might even find online tools to compress files.