The picture displayed by a user name is called an "avatar". To add an avatar, click the "User CP" (Control Panel) link on the upper left menu bar. Once there, you will see a vertical Menu Bar on the left. Near the top you will see a link called "Edit Avatar." You can upload any (decent) image you like from your computer, or from the internet. The only restrictions are 1) the image be decent (like duh), and 2) the image be no larger that 100 x 100 pixels.
If you find an image that you would like to use but is too large, send it to me in an email and I will resize it for you.