If you have the address of the pic you want to use as a banner...
Go to Control Panel (top-right of page) then Edit Profile.
Next to Signature paste
where
http://www.thisisbigbrother.com/foru...other-logo.gif is the address of the banner.
If you have a pic on your computer you can upload to something like imageshack.us or tinypic.com and get the picture address from there.