extreme_one It depends on the gif. Some work, some don't.
We've recently enabled some stricter automated checking that uploaded files are actually what they claim to be. Unfortunately, it appears that many gif images that are out there do not conform correctly to the gif file structure, and are thus failing our tests.
The same applies to some other image file types as well.
We'll take another look at the checking we're doing soon, to see if we can find a slightly less strict method that still delivers the security we're after.