How would you build visualping.io?

I'm most of the way through the Web Design track and as an exercise I'm studying other web applications and thinking about how I might build them. One I came across recently is visualping.io.

The source isn't too complicated. It's simple HTML and CSS for the front end. Looking at the source, I see bootstrap and YUI are used as well. What I'm lost on is what the back end looks like. Any thoughts about how you might do this?

They're not charging for the service (yet) or otherwise monetizing it so it can't be too expensive. It took 3 people to build it so I'm guessing it's more complicated to build than it looks. I see when the "Go" button is clicked it makes calls into an "api" directory, where it fetches an image. I can find a number of web services which allow you to fetch a website as an image, which they might be using, and they're probably using YUI to draw a box in the image to strict it to. A Quora answer suggested a program that can be run locally to generate images as well. Where I'm completely lost is, how are they comparing images. What does the cron job look like that looks for updates. Stuff like that.

So, community, what are your thoughts here? How would you build a service like this?