As an artist/illustrator, I've had people ask me if they can get my work tattooed on them, and I usually say it's okay. I have had people not ask me though, and i've somehow stumbled upon a photo of a tattoo of my original work without my consent. The way I look at it, is a tattoo artist should be okay with tattooing someone else's work so long as the client has the permission from the original artist. Of course, there are exceptions, like artists that have passed on or artists that are very popular and difficult to get a hold of. It's not really a black and white issue. All I know is I feel pretty ripped off when someone copies my work.