These small pictures are called avatars and are just a fun way to help identify yourself to other users. The picture can be anything you want as long as its not offensive. It must be a small picture (80 x 80 or less).
To add your own avatar, log in under your username and then click on "Profile" in the menu area. Scroll to the bottom of the screen and use one of the 3 options available to specify the location of the picture you want to use. You can upload the picture from your computer, from a website, or you can link to a picture on a website. Click Submit and your avatar will begin appearing on your postings.