Actually, infrared photographs show that the hands have been modified, and close-up photography shows that pigment has been applied to the highlight areas of the face sufficiently heavily so as to obscure the texture of the cloth. There is also obvious cracking and flaking of paint all along a vertical seam, and the infrared photos reveal in the robe's fold what appear to be sketch lines, suggesting that an artist roughed out the figure before painting it. Portrait artist Glenn Taylor has pointed out that the part in the Virgin's hair is off-center; that her eyes, including the irises, have outlines, as they often do in paintings, but not in nature, and that these outlines appear to have been done with a brush; and that much other evidence suggests the picture was probably copied by an inexpert artist from an expertly done original.
In fact, during a formal investigation of the cloth in 1556, it was stated that the image was "painted yesteryear by an Indian," specifically "the Indian painter Marcos." This was probably the Aztec painter Marcos Cipac de Aquino who was active in Mexico at the time the Image of Guadalupe appeared.
In 1985, forensic analyst John F. Fischer and I reported all of this evidence and more in "a folkloristic and iconographic investigation" of the Image of Guadalupe in Skeptical Inquirer. We also addressed some of the pseudoscience that the image has attracted. (For example, some claim to have discovered faces, including that of "Juan Diego" in the magnified weave of the Virgin's eyes-evidence of nothing more than the pious imagination's ability to perceive images, inkblot-like, in random shapes) (Nickell and Fischer 1985).
http://www.csicop.org/sb/2002-06/guadalupe.html