is the back seam straight up and down?  or is it curved?  i was doing one there for M3, and the sucker is curved :X

 

   with photoshop CS4 (or any with obj support, i suppose), you can create the tattoo as a shape or layer over top the 3d model, then 'drop' it onto the UV map.  robin wood has a tutorial on that:

http://robinwood.com/Catalog/Technical/PhotoshopTuts/PhotoTutPages/CS4-3D-4_Paint.html

 

   if the seam is straight, you can create the tattoo on a new layer -- two new layers, actually, and stick half onto the uvmap from either side.  but it's a tonne easier when you can see where it is on the 3d model at the same time, which is what i used to do with fractal painter 3d.  which came free on the poser 4 cd in ages gone by, i think.  (or poser 3, maybe.)

 

   modo can also drop (or shoot) a texture onto a model.  i dont know if blender can...

 

   what tools do you have for painting and creating the tattoos?