It is really easy, just copy and paste a snippet of code on your site and presto, you got this tool and DP got a link.
More information at Geo Visitors. How does it work? Well it snatches the image (so it knows what site it goes to) and then uses the referrer from people clicking on the image to pull up which map. Here it is for us at this site, just click on the icon;
Forum discussion at DigitalPoint Forums.