The question everyone wants to know, if it is not the work of an artist, then how was the image formed? Here is a summary of some of the theories.
http://www.shroudstory.com/natural.htm

Optical engineer, Kevin Moran discusses the phenomenon of how the Shroud image appears to be comprised of pixels, similar to a TV screen. Hence, darker and lighter shades are not the result of more or less "paint" but rather a greater concentration of dehydrated fibers, ie. Pixelization. Note: This takes a minute to load and contains 8 images.
http://www.shroud.com/pdfs/moran.pdf

Does the Shroud contain three dimensional data? Pete Schumacher, inventor of the VP-8 Image Analyzer first used in 1977 on the Shroud reveals that the shroud image acts like a spatial data base, similar to a topographical map. The following paper is 8 pages long and contains images.
http://www.shroud.com/pdfs/schumchr.pdf

More on the VP-8 and the 3-D image.
http://www.shroudstory.com/vp8.htm