On the upper-center part of this (or any) page, click on "Profile." Then, on the left side of that page, click on "Forum Profile Information" under the "Modify Profile" section.
From there, you can choose from a variety of pre-uploaded images, or you can link to an image you have uploaded somewhere else online OR upload your own image. All of those options are in the middle of the page.
Hope this helps!
Brent