They're practically broken unless you're lucky.
If I remember correctly, SMF added an update that changed the way they check for viruses in the images uploaded or something like that, but it was way too strict and so a lot of avatars get rejected for no reason because of the image header, and they won't change it back.
Or something like that. I'm too lazy to just use the search button and find the exact post that links to the SMF thread.