It must be some problem because it's 2004 now.
Well, I haven't tried changing my signature or avatar yet, but what I'm guessing is it's a problem with the frost and emerald servers. I guess if you want to change an avatar, maybe it'll work changing it on this board, which is on the bb server. Your avatar/signature will probably be the same on the other boards, but changed here.
You could also try again later. I had an avatar frozen problem once. A few days after it wasn't changing when I wanted it to, I just went into Edit Settings in User CP and backspaced out the link to my avatar, clicked Save Changes, went back to Edit Settings, and typed it back in. It worked.