First, you need to have a photo on your computer. It has to be in JPEG or GIF format, under 75x75 pixles in physical size, and under 20KB file size.
Next, you go to your "User CP" on this this site. Click on options once you get there. Scroll down and you'll find the Avatar section. Click "Change Avatar". Now that you're there, click the "Select avatar" button and find the file on your computer. Then click the "Use custom avatar" button. Hit "Save Changes" and that's it. You should have your picture (in my case, a Jonsereds logo) under your name.