Well if you upload a new avatar, it replaces the old one, so it's not a problem with the forums (since it can't display something that's been overwritten). Same with avatars that use a specific external url rather than uploading one.
I'd say it's down to the browser. Try clearing the cache completely, and then restarting the browser.
I'd say it's down to the browser. Try clearing the cache completely, and then restarting the browser.