This photo is old, I've since recast in clay and resculpted the head since my first mold was too flawed. But it should give you some ideas.
The problem with reference photos for Ryuk is that the bugger is different every time he is drawn. My chief reference photo was the one on the inside cover of the first manga...You should just pick one you like, and focus on it more than any other. To be more specific, I purchased the Death Note art book, which is gorgeous. I also bought a bunch of the prints in poster form and hung them up all over my sculpting station.