On the main page (go to the top of this page and hit the upper left site logo) look for the line of blue words. One says 'Photo'. Click that then follow the instructions given on that page.
Keep in mind this picture will only appear on your profile page, and not in the forums.
If you are looking for the picture that shows in the forums when you post you want to go to that same line of blue words and click 'Avatar', then follow instructions from there'.