The statement that it is a photographic negative is false. The hair, eyebrows, and "blood" are not negative in the picture. It is a fake shroud, not a fake picture. It looks somewhat like a negative because the high points of the body that come into contact with a shroud are darker than those further away. A painting has closer features lighter. It isn't anatomically correct either. One arm is longer than the other and the hair forms a nice little cap and isn't flattened out. Also, the character on the shroud conveniently has his hands over his winkie as if anticipating that the "shroud" might be shown in church. This was common in medieval painting, but Jews were buried with their hands crossed on their chests.
The factoid that the Shroud is a "photographic negative" is a convenient popularization of what scientists actually know it to be. In fact, the lack of light or photographic artifacting is one of the major mysteries of the Shroud. There are not shadows or other light based image characteristics. The image on the Shroud is a quasi-terrain map in which the intensity of the image forming "pixels" is proportional to the closeness of the body to the cloth. This gives the image an appearance of being a "photographic negative" when it is not.
Again you raise a false strawman argument, knowing it has already been shot down by scientists examining the Shroud over the past 110 years since Pia Seconda took the first photograph of the Shroud and noticed the negative qualities.
That still does not invalidate Joe's point that any medieval artist would have to first conceive the concept and then work his art, perfectly, so as to "fool" 20th and 21st Century science. It ads a level of complexity to making such a fake... and also raises the question as to why such complexity is necessary to fool peasants into giving up their hard earned sous and ecus.