These small images are called Avatars. They are displayed below your username on all posts that you make. There are two kinds of avatars: those provided by the administrator and those that you upload yourself.
If the administrator has provided a set of avatars, and avatars are enabled, you may select an avatar that best describes your personality.
The administrator may also have enabled custom avatars, which allows you to upload an avatar image from your computer.
To edit or add a user avatar go to the User Controls "user control panel" and click Edit Avatar. Upload any image and it will be sized properly for you. You are then DONE. Now every time you post you will see and Avatar below your username.
See Below to see what an Avatar looks like.