The puzzle works by having a number of <img> with ids that encode the co-ordinates of a grid. The javascript processes clicks on the img elements and swaps over the src of the img when necessary. An onLoad handler is used to scramble the images when the page is loaded.
Here is another larger puzzle!
I'm thinking about a cgi page that lets you upload your own picture and sends you a zipped set of files that make up your own puzzle... coming soon?
-Cam