You could either use your favorite paint program and draw or add a layer with the tattoo directly onto the Skin Diffusion map...maybe the Upper or Lower map etc.
Or what I prefer, and you'll be able to get higher-rez, would be to use the Blend Map channel. Make sure the image is on a white background with a lot of white space around the image then you can place it using the U V Offsets to position it on the body. With a lot of white space you won't run into a "repeat" of the tattoo.
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