Those are avatars. If you logged in and click on user list under your user menu , you will see the list of users and avatars representing those users. You can either choose from existing avatars or upload your own. Under user menu click on "Your Profile" link then click on shaded EDIT link. You will see then 3 links to update your profile, update your image or delete your image. Click on update your image and you will have options of choosing existing image from the list by checking the chosen image and then clicking on "confirm choice" button on the bottom.

You can also select file on your computer by clicking on "browse" button on top of the page and directing it to the picture on your hard drive. Picture has to be no larger then 120x120 pixels and no larger then 100kb. You can use any picture editing software to downsize the picture before you try to upload it as your avatar. Once your picture is uploaded, you will see confirmation on your screen. You will not see the picture right away in your profile , because it has to be aproved by administrator which takes from couple of minutes up to couple of hours, so be patient.  By the way : No porn pictures please!!!