To quote myself from the last time this question came up (yes, it's been asked at least this many times; please use the search function before posting to see if your question has already been answered!):
Any progress?
Please turn off this forum! The avatars are the only advantage over a mailing list. This is bitter but true. Give something better a chance. Give up!
I'm pretty sure that avatars will be disabled until at least the new forum is out and being used - as theymos has stated it was removed due to an exploit made possible by avatars and though I believe the exploit has been patched he is not keen on restoring the functionality.
From what I've heard, pretty much. The exploit involved uploading a php script instead of an image into an executable directory, then using it to put some (slightly) malicious Javascript into the forum's MOTD so that it ran on every page. I followed the troubleshooting and analysis through IRC while it was being fixed immediately after the hack, but it seems that avatars may be disabled for a while anyway.
Though the exploit used has been patched, it's been long enough that I suspect new avatars may remain disabled until the new forum software is put into place, but those who had them before the hack like me were able to keep them (though we can't change our avatars either).