If you want a banner in the post itself, you are going to have to post it automatically.
However, I believe you are talking about something like my Frodo banner. If you are, that is called a signature. What you do to get on is: go to the "Profile" link up top and head down to signature. When you reach the box, you are going to insert this code:
Code:
[img]URL OF IMAGE[/img]
For the URL OF IMAGE, you can go to an image hosting site such as ImageShack, Tinypic, or Photobucket, and it will provide the url for you.