If you have your skin textures, and found nice tattoo textures, you can make a new skin textures set with the tattoo merged in it. You don't need to use Photoshop or Gimp, you can do it with applications online, and very quickly. This articles will show you how to do it, even if you do not have editing skills. You just need the textures saved in PNG format on your computer.
There are many online image manipulation applications, this works with any application that can provide layers. I chose for the demonstration Pixlr because it's powerful, free to use, and easy to use. Go to the site and launch the application:https://pixlr.com/x/
Load your skin texture
First click on the Open Image button and go browsing for one of the skin textures. Let's use the upper part of the body (all the steps will have be repeated for the other skin textures needing merging).
Load your tattoo texture
On the right, you can see Layers and at the very bottom right, a Plus button. Click on it and select the Image type :
Then go browse the tattoo texture. Things already look good !
Tweak the layer
This step is not mandatory. If your tattoo looks good, then skip this and go saving your work.
Once you have loaded the second layer, you can modify it, using controls on the left of the application :
The most useful ones for beginners are the Transparency setting and the Blend Mode. Play with them to find nice effects :)
If you get lost and want to return to normal, use Undo button at the bottom of the application.
Save your new texture
It's time to save your work. Just click the Save button at the bottom of the application. The important thing is to use the PNG format instead of JPG, because JPG tends to alter the quality of the image. The best for OpenSim would be to save in TGA, but this application does not support this format, and PNG is quite good too :)
Do the same for the other body part (lower texture), and if you have a tattoo that goes up to the neck, do it for the face texture as well.
Import in OpenSim
Upload the two (or three) textures and use your favorite applier.