Any aspect of the law in the US or elsewhere can get very complicated and this is no exception. If you're really interested in what the US law is and the fair use doctrine, here is a fairly good plain language presentation from the US Copyright Office: http://lcweb.loc.gov/copyright/circs/circ1.html
Having said that, IMO, I agree that you should make every effort to contact the holder of the copyright when using a trademark or copyrighted image. From a practical standpoint, I wouldn't let that stop me or worry me overmuch in most cases of skin art. The real reason people are sued for copyright or trademark violation is when:
A) You profit or attempt to profit by selling goods which use the copyrighted image or trademark without consent of the holder of the rights.
You use the copyrighted or trademarked image in such a way as to state or imply an association with or endorsement by the holder of the rights which does not exist. Profit or attempt to profit is not a necessary component of this offense.
C) You use the copyrighted or trademarked image in such a way as to reflect discredit on the holder of the rights or in a manner which would damage the reputation, image, or marketability of products for sale by merchants using the image with the permission or license of the holder of the rights.
I'll grant anyone who wishes to argue the point that these are not the only reasons why you may be sued or the only possible violations of US law in this area. But for all practical purposes, these are the main areas in which actual damages can be assessed (money calculated) and that's the main reason you get sued. Beyond that the worst that will normally happen is being ordered to remove it from public display.
I hope this helps clear things up a little.
