Getting artwork to look dimensional is one of my favorite things. I've always been fascinated with Trompe L'oeil. Trompe L'oeil is a French term that means 'to fool the eye'. If you google it will will find some pretty amazing stuff.
Trompe L'oeil is all about getting the shadows and shines in the right places. I know this tile isn't quite there (I can find all sorts of goofs) but it's fun to try.