They are called Avatars/Displays Pictures/Icons whatever. On this site we call them "Avatars".
They are a small picture that you choose, and change as you wish. They are not based on "rank" on anything like that, any member can have one.
(you can find various profile and member control like this in your "Control Panel" which can be access by clicking the "cp" at the top left)
Edit Your Avatar Here
We have a selection for you to choose from, or you can upload your own.