You could do like some other people do:
Create a database with the file names of the banners, and the URL it should point to. Then you make a cron job on your server that renames the "imagefile" to live-banner.jpg every x minutes and matches the redirect script with that image-name.
Then you change your signature to use the
www.server.com/live-banner.jpg and link it to
http://yourserver.com/redirectscript.php
That way you use the same static code, but the work is being done on your server