Are you looking for images to change when refresh the page or automatically change every few seconds? Either way youll need to modify the portal.tpl but will only have to do it once. Where you placed the code for the image use a php includes so you only need to edit that file once to change images.
Replace the image code you added with something like this
{php} include("/path/to/includes/script.php"); {/php}
This means you'll only need to edit the script.php or what ever you name it.php to add or remove images and links.
If your wanting it to change when you refresh page you would add some php code to the script.php or what ever you name it.php like this.
<?php
$banner_src[0]="http://www.yoursite.com/image.jpg";
$banner_link[0]="http://www.yoursite.com/desired_link";
$banner_src[1]="http://www.yoursite.com/image.jpg";
$banner_link[1]="http://www.yoursite.com/desired_link";
$banner_src[2]="http://www.yoursite.com/image.jpg";
$banner_link[2]="http://www.yoursite.com/desired_link";
$banner_src[3]="http://www.yoursite.com/image.jpg";
$banner_link[3]="http://www.yoursite.com/desired_link";
$banner_src[4]="http://www.yoursite.com/image.jpg";
$banner_link[4]="http://www.yoursite.com/desired_link";
$banner_src[5]="http://www.yoursite.com/image.jpg";
$banner_link[5]="http://www.yoursite.com/desired_link";
$banner_count=((count($banner_src))-1);
$which_banner=(rand(0, $banner_count));
print "<center><a href=\"$banner_link[$which_banner]\" target=\"_blank\">
<img src=\"$banner_src[$which_banner]\" border=\"0\"></a></center>";
?>
Now if you want it to automatically change images you'll need some java scripting or do a flash image. Just replace that code with the php coding in the script.php or what ever you name it.php file.