What you need to do is wrap the URL Tag around an IMG Tag of your banner (instead of wrapping the text). For example, in your signature:
[ url=http://www.google.com/][ img]http://www.google.com/intl/en_ALL/images/logo.gif[ /img][ /url]
Will generate:
Note: There are no spaces between the [] brackets. I only put the spaces in so that the code would be visible. Here is a useful
BBCode Guide that may answer any other questions for you.
Dominixe