You should get a 3D painting program.

Have a look at this thread (page 2)

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2716576&page=2

The thread was original about how to make a tattoo shader look real. But then somebody asked the same question - how to perfectly split and line up two parts of a tatoo across a back seam.

I used the free version of Blacksmith 3D to just click it onto the figure. The program let's you paint right on the figure (or slap on images using an image brush) and it re-maps it onto the UV map of the figure, across material zones, or whatever is necessary. It handles all cutting, rotating, and scaling automatically.

You may find the rest of the thread interesting as well. I show how to set up shaders for tattoos.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)