This is a picture of a real human skull I photographed some years ago for a project I never completed.
Download the zip file for the RAW format version (MRW which I think stands for Minolta RAW and opens in Photoshop etc.)
I borrowed it off a guy for a day and it was slightly odd cycling round town with this thing in my backpack. I was thinking that if I got stopped and searched by the police it was gonna look pretty damn weird. Don't know much about the skull except that it was pretty old - ie. some hundreds of years as I remember.
I'm not really into rules. Use as you want on or off DA. Post a link here to artwork if you want. I have a load more pics of it which I'll upload some time especially if popular.