I would like to quote Michaël Pierard here:
Most bloggers monetize their blog, do you? If the answer is yes, you probably grew frustrated having one or more 125*125 pixel advertisements. Most people use a text widget with some handmade HTML/CSS to accomplish this. While this is a good way to do it, you're wasting a lot of your precious time.
ISIS Ads Management provides a super clean interface to manage and display 125*125 pixel ads.
Get your copy of the plugin at http://www.catswhocode.com/blog/featured/isis-ads-management-new-wordpress-widget-to-displaymanage-125125-ads.
Once you have downloaded the plugin, follow the instructions to install a plugin on page 1.
When ISIS Ads Management will be successfully installed, you'll see an ISIS Ads Management tab under Manage on your WordPress dashboard.
To utilize this recipe, you'll need a set of 125*125 px ads and their relative links.