Since our shoulder tattoo picture mixer design was getting some interest, we've added a more powerful back tattoo design. It looks like this:

You can move your tattoo around the body and scale it as you like. Just prepare it as a "png" image with transparency, upload it, then work on placement. This way, you can get an idea of how the tattoo might look on you.

The body in the design is from the excellent makehuman project.