Michelene was right with the definitions of citizen, etc.

The picture under your name, though, like mine of Miss Hoppy, is known as your "Avatar." You can use any picture that is 60x60 pixels or smaller, that you have saved on your hard drive.

Just go to "User CP" up at the top. then once that pafe loads, hit "Edit Options." At the bottom of that is a place that says "Avatar" and at the right, "Change Avatar." Hit that, then hit "Browse." show it where and your hard drive the image you want to use is, then you judt nrrd to save it, an you're done!

The Signature picture would appear below your posts, not below your name, and that's a different process.