that isn't always the problem but it's definitely worth a shot if you're having trouble
like, I was having tons of problems uploading my avatar(s) for valentine's day a few weeks ago, and among the ones I was having issues with, was a regular .png from paint.net
when I uploaded this one, it worked, but it was still a regular .png from paint.net
I recall getting an error about the size of the file, like the one they mention in that thread, but I didn't get any error at all this time