There are gremlins running around this web site. Sometimes there are windows of opportunity to edit your avatar. I just got lucky when I uploaded mine. I know I dare not try to change it. It's just like how some days you have the option of deleting a post and some days you can only edit a post. I don't know for sure why these inconsistencies exist but I always just assumed it was due to system maintenance or some other IT-guy related reason. Somebody is adjusting the web site, setting the controls for the heart of the sun, or some such wizardry and they flip a switch in the process of doing so that temporarily allows access until they notice and come back and switch it off again. That's just conjecture on my part. Don't take it as gospel. The point is to keep trying from time to time. Maybe lightning will strike.