Ive found that decals work good. Especially when crossing over a seam like the back. Id pose first then add them.
The issue with tats is a lot of them ive found dont blend with the skin. You need to drop the transparency a little.
Ive added a few in PS and that works. Also tried geoshells and that works.
Decals are a bit more forgiving because you can move them around easily and resize easily. Not sure how well they work in animation. Would need to parent it.