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 forum owners and those that you upload yourself.
To add an avatar to your account, go to your User CP and select "edit avatar" from the menu at left, then follow the on screen instructions.
Posts by members, moderators and admins are not considered medical advice and no guarantee is made against accuracy.