NOTE: Although lou133 has already explained much of what I'll be saying I figured out you really seem lost judging from your post. SO I decided to try to explain it in a bit more details.
So you would like to have a picture displayed beneath your name? Is that it?
If so, doing so is quite simple really. At the top of the webpage, there should be a task bar with menu titles: HOME, HELP, SEARCH etc. Just hit the one labeled 'PROFILE' which will allow you to gain access to all information pertaining to your... profile. In there, you will find a menu on the left where you should click on 'Forum Profile Information', under the heading 'Modify Profile'. The page that will be loaded will provide you with the means to select an existing, or upload your own avatar/display pic. Just save the changes, and voilà.
I have read in a post that not surprisingly, some people don't like others using rank/trade/organisation avatars if this information does not actually reflect their standing. Keep that in mind if selecting one of the avatars proposed to you.