Go to user settings at the top. Click on edit options. Scroll down to the bottom where it says avatar. Make sure your image is 70 or 80 pixels in size (I think) and attach it.
I'm curious about this one too. I've tried to post an avatar dozens of times with no success. I keep getting a message saying "this image is too large" despite the fact that it's 75 x75 pixels and under 15000 bytes.