it's part of creating your profile. go to "My Profile". all the way at the bottom, there's a section for loading your avatar (picture. why they can't just call it that is beyond me, but that's all it is).
click on the "browse" button and go through the folders on your hard drive until you find the one you want to use. highlight the name of the file, then click on "open". once you get back to the profile screen, click the "submit" button. as long as your picture is small enough (note the size restriction stated in that section) you'll have no problems.