wouldnt that open up the doors for anyone with a tattoo of say any of andy warhols work to be sued for enfringement?...yes thats the backwards version of this argument, but you know that it would happen.
Technically, if instead of a tattoo it were a painting, the artist would get a cut if their painting was used in a campaign. But one would be inclined to see how major car manufacturers were reimbursed when it comes to using them in movies and adverts. especially after a jesse james or doug foose gets ahold of them.
Personally, I think he does have a right to the artwork itself -in that he can 'sell' (ie tat) it again- regardless of the fact that its on becks bod. However, in this case, if it werent for beck no one would know who Molloy was. It's a catch 22, just put a caption on the bottom of the pics stating that Molloy is the artist and leave it at that.