The answer depends on whether or not you have a layer mask for the tattoo. If you don't have a layer mask, then the answer depends on whether the tattoo layer has a unique color for the "not tattoo" parts, such as black or white.

Also, as ashley9803 suggests, I can do it with nodes instead of Photoshop. I like the node approach because:

1. I can skip all that workflow associated with switching between Photoshop and Poser, saving the image and doing test renders. The material room preview will show me exactly what I'm going to get.
2. I can size, position, and rotate the tattoo in shaders.
3. I can easily layer many tatoos in the shader, giving more options for combinations.

How do you want to proceed? What's your scenario?

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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)