That's called a signature. You can change your signature by visiting your profile page and hitting the "Edit my profile" button. Hitting this button will take to your User CP. On the left hand side, you'll see a couple settings tabs. Hit the "Signature" tab. From there, get a link to the banner that you want to have in your signature and hit the little image button in the editor. From there, paste the link to your banner in the input box and you're done.