Is it 75x75?
If your .gif is any larger than 75x75 at all it will get converted to the first frame of it crushed to 75x75.
I've tried an online resizer before but I can't recall what it was called.
I haven't tested this one out I'm afraid, but it claims it can do 75x75, so it's worth a shot.
Just make sure it is 75x75 or under.
http://www.resizeyourimage.com/And other than the scale, I'm not sure.
I'm not sure if there is a limit on the data size of avatars though. And if there is I'm not sure what happens if it's too big. I would imagine it wouldn't accept it.