Hey Beardo, there's a few things I can suggest you check (if you haven't already):
- is it 100x100, or 150x100 if you own the addon (it will lose it's gifness if it has to be automatically resized)
- is it smaller than 683.6 KB? If not one solution is to cut a few frames off to make it smaller
- do you own the gif avatar addon?
If you still can't get it to work link me via imgur and i'll have a look for you