I don't know if it can be added here or not, but it looks handy to have a place with not only the tips & tricks but also some free tools, such as the scripts that I have developed for Shana's Tattoo sets. And it can be used for your own tattoo or whatever bitmap you would like to us to overlap any skin part.

Here: https://www.renderosity.com/forums/threads/2991817/poser-tools-for-shanas-tattoo-sets

For example Rhia4764 used this tool to add freckles, scars, etc...
Of course, the script is free and does not contains Shana's tattoos.
It's written for Python 3.7, so that it should be usable with Poser12 at least.
Freshly tested on Poser14:

If its place isn't here, a mod can delete it without problem 😊

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