If you want something super basic, you can use the code below. All you need is a plain text file with the link and image tag in the file, one per line. Call the file ads.txt if you want.
PHP Code:
<a href="somesite.com"><img src="ads/banner.jpg"></a>
then whereever you want the banner to be, paste this code in.
PHP Code:
<?
$items = file("ads.txt");
$item = rand(0, sizeof($items)-1);
echo $items[$item];
?>
Like I said, its basic but its functional.