Lots of people have been asking about this lately. Right now the "custom avatar" feature is not working through the forums. In order to change your avatar, you must you Gravatar.
Sign up for
Gravatar (it's free) and you can change your avatar that way until we get this issue fixed. Give it a few minutes to kick over from Gravatar to the forums, as it can take some time (5 or so minutes).
Thanks! If you have any questions about this issue, please post on this thread.