I've been there and tried it it keeps saying url invalid. I use imgur and followed IIona's instructions but it wont work.
I was logged in yes. see if you can change it now. warning you might be stuck without an avy for a while if you mess with this one.
The old forums has been shut down so i'm thinking the system just does not work anymore. i'm hoping the staff can work something out eventually. maybe they didnt realize this would affect something as vital as the pure joy of tinkering with your avatar and now its just gone forever lmbo !
what do you use if not imgur that could be the problem idk