Those pictures are called avatars. You can add or change your avatar from your profile page. To access your forum profile page, click on the My Stuff link at the top of the page and select My Profile. Near the bottom of that page, you'll see the section for changing your avatar. You have the choice of removing your current avatar, uploading your own image, linking to an image on a web site, or using a stock avatar from our site. Follow the instructions that are presented to complete the selection of your avatar.
You can upload any picture, but be aware that the forum software will resize the picture to 80x80 pixels. If you want to use a screen shot from one of your games, you can see how to do that
here.