@theMezz-This post is a little dated but oh well ??
A quick google search brought these two up, give them a look:
Plugin 1
Plugin 2
This could also be done pretty quickly if your comfortable placing some php in your sidebar. A super quick example, not tested:
<?php
$randomNumberAd=rand(1,15);
if($randomNumberAd <=5 ){
$randomAdUrl = '<img src="path/to/your/image1" alt="Img 1" />';
}
elseif($randomNumberAd >= 10) {
$randomAdUrl = '<img src="path/to/your/image2" alt="Img 2" />';
}
else {
$randomAdUrl = '<img src="path/to/your/image3" alt="Img 3" />';
}
?>
<!--Usage Below-->
<div class='yourAdClass'><?php echo $randomAdUrl; ?></div>
Best Regards,
Drew