Google Ed Hardy and you will get tons of old school tat graphics. I would use melted chocolates to pipe the tattoos and attach to the sides, etc. I think it will give the depth and detail you are looking for and this medium is sturdy and consistent, albeit a pain in the butt to keep the chocolate warm but worth it, I think.
Sounds fun, I would love to do one of these cakes..... DH's b-day is in Nov. and I think I just figured out what the theme of his cake will be! Thanks