The difference is that the Gifs are resized, JPEGs are not. I don't know why this is. If I have a chance I might play with it and see if I can make it stop resizing the JPEGs.
I think it resizes JPEGs because it can, and that's not necessarily a good thing.
If you use a 20x20 gif, I think it will work
In any case please don't post a 1000x2000 user image. it would take most users 5 minutes to download it and I doubt they would hang around that long
Regards,
Matt