Lots of cool tattoos on the market, but they are either psd format or come attached to a much less than desirable skin texture,
I  think there are plenty of people like me who want to apply tattoos to favorite textures  but don't own Photoshop, don't want to pay for it and don't want to learn how to use it. Between the price of Photoshop and the class I'd have to take to learn it, that psd tattoo selling for $9.00 would actually cost me about a grand.
Gimp, you say? Have your tried to download Gimp lately? And the 52 extra programs required to install and learn it? And the the download links hidden like a needle in a haystack amongst adds and phony download buttons that infect you with spybots? A curse on Gimp and its makers who act like its free and then whore it.
Right now I can hit a button and apply stockings to my favorite body textures, why can't that be done with tattoos?
Oh, and elaborate, color tattoos that cover lots of skin.

There's GimpShop (www.gimpshop.com, www.gimpshop.net). It's a Gimp derivative, without the infamous Gimp download and UI hassle. But with the Gimp price tag: it's free.
It can imporrt and export .psd files, so you can use those tattoos. It can also import/expord PSP files, .png and .jpg, amongst others.

You just need to learn the trick with nodes to overlay one image on another .  

Daz Studio is free...easy to use and it has a plugin program that's called Layered Image Editor that does exactly what you want ......adding not only tattoos but makeups and second skin stuff...etc.  You can find it at Daz

Thanks guys, all wondeful suggesions! May all your hopes and dreams come true!