Adding your own ads is easy- all you need to do is open up sponsors.php in the theme directory and you will see 4 lines like this:
<a href="#"><img src="<?php bloginfo('template_url'); ?>/images/ad.gif" alt="Sponsor" /></a>
Each line is an ad image/link.
Change the “#” to the URL the ad should go to when clicked. Add your 125×125 ad graphic to the theme images directory and then change “ad.gif” to the name of your ad.
A completed example would be:
<a href="https://www.google.com"><img src="<?php bloginfo('template_url'); ?>/images/google_125x125.gif" alt="Sponsor" /></a>
This ad would show the google_125x125.gif image located in the /wp-content/themes/wp-portaltheme/images/ directory and link to https://www.google.com when clicked.
Optionally, you can change the alt text that says “Sponsor” above to something more search engine and accessibility friendly.