Your accounts page (the one mentioned by Snowman) might not work.
Try using the one on joeuser.com . Go to joeuser.com. Sign in using your nickname and your password.
Then in the bar across the top of the page, go to "My Account" and then choose "Images" (which is in gold color font). Under "My Account Images", the top one should be "Avatar".
Upload your avatar. It should be "a *.JPG (.jpg), *.PNG (.png) file no larger than 100 x 100. Maximum filesize is 40 kb."
That should take care of it.