The advanced search page at IAFD has a tattoo section. You can specify what's in the tattoo or the location. It's a little vague but mostly functional.
For example
butterfly AND back
Would bring up chicks with a butterfly tattoo and a tattoo on their back, but not necessarily a butterfly ON their back. Still, it's pretty nice and fairly comprehensive for porn stars. I've used it to cross reference search for chicks here a bunch of times.
I wanted this feature for ages and was so disappointed with what was out there that I build one myself!
I expanded the site to search for all physical characteristics of a person, from tattoos to scars, moles, hair colour, piercing and a whole lot more.