CreateMockup is powered by WordPress Mshots. Mshots is a Website Thumbnail/Snapshot Service created by Automattic. If you want to run your own Mshots service you can go the the original Mshots Github source. You can create unlimited web screenshots and the maximum size is 1024px without watermark. That’s pretty awesome!

Example url: http://s.wordpress.com/mshots/v1/http%3A%2F%2Fcreatemockup.com%2F?w=250