It's pretty easy, just include a bit of PHP in your template index.php where the banner is displayed and then call the appropriate banner based on the page_id. You can have as many different page/banner combinations as you want.
For example, I use this approach to include some java script on my pages...but only for those pages that use the JS. Here's my snippet (you'll need to modify to suit your needs by using an <IMG> tag):
<?php if (($page_id==27)||($page_id==29)||($page_id==34)||($page_id==37))
{echo "<script type=\"text/javascript\" src=\"/js/general.js\"></script>\n";}?>
You would change this to basically something like "if page_id=X then use image Y otherwise use image Z"