I was actually wondering that myself but haven't had a chance to ask the artist...
Wolverines. No, I am not a U of M fan. I am however a Red Dawn fan.
I tried a technique I thought of. It might not be original, but I am self taught and have to figure this stuff out myself. I overlaid a photo of the Wolverines graffito, traced over it, then "pulled out" the photo. It didn't work quite as well as I had hoped, but I think it may just need a little practice. I do plan to try that part again when I have a little more time.