mystic...  postwork?  like not on the skin, just on the render?

 

   i've had some success putting a tattoo on a figure in photoshop, using shapes. 

http://bloodsong13t.deviantart.com/gallery/34735447#/d4ph2iv  (contains nudity, so probably wont display if you dont have a devart account.)

http://bloodsong13t.wordpress.com/2011/12/13/zevrans-tribal-tattoos/  (will work too)

 

   it's do-able; depending on how much shape-manipulation your version of photoshop has.  for example, you could make all those butterflies a new shape or raster layer very easily.  but look at the 2nd one down.  it has foreshortening where it curves into the dip of the spine.  so you'll have to 'smush' one of its wings.
     you always need to 'bend' the overlying tattoo to follow the surface of the skin.  unless it's a really straight-on shot.  you'll also need to experiment with the blending of the tattoo layers and the underlying skin.  when i was doing zevran's tattoos in the example, i found it worked best to get the ink UNDER the skin by erasing down parts of the tattoo layer that were over highlights on the underlying layer.  i was pleased with the results of that.