Small pictures underneath your username, commonly known as "avatars," are, at the administrator's discretion, available for use by certain or all forum members. Generally, avatars are in a .gif or .jpg format. It is common courtesy to use small avatars so as not to change the size of any tables or mess up the look for anyone viewing the forums. Also, keep in mind those using slow internet connections, and don't use an avatar with a large file size (bytes).
To add an avatar, simply go to the user cp located at the top right hand corner of almost any screen, click on profile, and down the list, you will find "Avatar URL" where you can enter the url or web address of the image. The image will not work off your hard drive, so you will need to upload it to the web for it to work correctly. Simply click submit for the settings to take effect.