The way you could do it is find your banner, edit it in photoshop, or some such editor, and write the description you want on the image.
Upload the image to your images directory.
Then in the forum description, replace what you wrote there with this:
Code:
<img border="0" max-width="XXXpx" max-height="XXpx" src="/images/MYIMAGE.PNG">
Changing,
MYIMAGE.PNG to the name and extension of your image, and change the
max-width="XXXpx" max-height="XXpx to suit your needs