I may be wrong, but I think you have to apply the tattoo in a graphic program to the texture file and then save it. You would want to save the .jpg as a different name though, and then get MAT Pose Edit in order to create the files you need to use. At least that's my understanding of textures. I haven't made any other than for a hand fan, but that's what I had to do. I don't know if skin textures work differently or not.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi