30 seconds guess: Author got hold of a large set of photos, analyzed for raised fingers with computer imagery. Then author probably split the 960x680 px rectangle into a grid of small squares. Located the finger in every photo and assigned each of them to a specific square.

From there on it's a matter of finding the pointer in javascript and a Voronoi search to the matching photo. I'm not seeing an ajax call in the logs so there's probably a bit more magic going on clientside.

Edit: Terminology