He used a picture on his post by Street Artist Edgar Mueller. I think he's an extremely talented artist, not just for his ability with a bit of chalk, but for the ability to get his angles spot on when looking from a specific direction. He drawers on pavements and elongates the image so that when you stand in front of it, it looks in proportion and almost 3D. Here's an example.

I couldn't find a direct website for this guy, so instead, here's a link to the website where I found these. Having said that, you could always Google Edgar Mueller and check out the images...
Enjoy and have a good weekend!
