It has to be a 75x75 or a 90x90. I keep forgetting. Cybersix sized my avatar for me, so if you need help ask anyone
75x75 is the requirement.
If you have one that's any larger than that (even a single pixel in 1 direction) then the image will be converted to a .jpeg of the first frame of the .gif
You can use a .gif resizer to change the one you want to 75x75.
If it's got an unusual aspect ratio, then you can just shrink it down until both the height and width are 75px or less.
So, if it's 100x80, change it to something like 50x40, or what have you.