We have become aware of a security issue with the forum software used here that might allow a malicious user to harm the site by uploading a specially crafted avatar.
This potential exploit definitely has not been used, and we have no reason to believe that any of our existing users have malicious intentions, but as a precaution the uploading of user avatars has been temporarily disabled.
We expect a future version of the forum software will address this issue, at that time user upload of avatars will be re-enabled.
In the meantime you can still add or change an avatar by using a URL that links to an avatar file hosted somewhere else.