Does anyone know how this was implemented? It's a great example of a case where, as a computer programmer, there are still problem domains which I have absolutely no idea how to approach!
[1] http://www.pointerpointer.com/gridPositions.json
Try moving your pointer around to different positions very close to the top right corner. This triggers a picture with a wristwatch and water bottle. The pointing is done by the right hand, which if your pointer is too far right is cut out of the frame, but becomes visible as you move towards the left.
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
For a small number of photos (up to a few thousand), a linear search (i.e. check against each one) would be quick enough. You could compare the distance of the pointed spot to the cursor (using Euclidean distance), pick the best one, maybe add in a random factor.
EDIT: Somehow the author is putting Voronoi diagrams into use. Take a look at the source.