If I were you I'd just forget about it. I tried with several file types... no bigger than 19kb, but I was unable to upload an avatar. So I gave up and moved on. I advice you to do the same, because it looks like this is an issue linked to the forum.
Thanks, Jonny Glad my comment was of help, hopely the admin sees this post and decides to do something about this. So people can finally start adding avatars