Similar to the Shroud of Turin, the tilma image also defies scientific explanation.
As I have stated before on Shroud threads, I follow the science. There is very little science associated with the Virgin of Guadalupe image. Unlike the Shroud, the Virgin of Guadalupe image has not undergone extensive, or even more than cursory, scientific examination.
Several Shroud researchers have examined the image of the Virgin.
"Norberto Rivera Carrera, Archbishop of Mexico, commissioned a 1999 study to test the tilma's age. Leoncio Garza-Valdés, a pediatrician and microbiologist who had previously worked with the Shroud of Turin, claimed, upon inspection of photographs of the image, to have found three distinct layers in the painting, at least one of which had initials painted on it. He also stated that the original painting showed striking similarities to the original Lady of Guadalupe found in Extremadura Spain, with the second painting showing another Virgin with indigenous features.  Garza-Valdés also claimed that the fabric on which the icon is painted is made of conventional hemp and linen, not agave fibers as is believed. Gilberto Aguirre, a colleague of Garza-Valdés who took part in the 1999 study, examined the same photographs and stated that, while agreeing the painting had been extensively tampered with, he disagreed with Garza-Valdes' conclusions and claims the conditions for conducting the study were inadequate.
Garza-Valdés, is the Shroud "researcher" who claimed the 1988 Carbon testing of the Shroud was invalid because he found a "bio-plastic' coating on the shroud fibers left behind by generations of bacteria. This hypothesis was easily discredited. I don't give much weight to Garza-Valdés, findings, but if the base material is hemp and linen, as he claims, then the miraculous survival of the agave cloth image is no longer miraculous because hemp and linen can easily survive 500 years, unlike agave which decomposes fairly rapidly.
Another researcher of the image, art restoration expert José Sol Rosales (not, as far as I can determine, a Shroud researcher), who examined the image of the Virgin of Guadalupe under a stereomicroscope in 2002, says that he "identified calcium sulfate, pine soot, white, blue, and green "tierras" (soil), reds made from carmine and other pigments, as well as gold. Rosales said he found the work consistent with 16th century materials and methods."
"Richard Kuhn, who received the 1938 Nobel Chemistry prize, is said to have analyzed a sample of the fabric in 1936 and said the tint on the fabric was not from a known mineral, vegetable, or animal source."
I have stressed the word "said" in this report because no one can find any primary citation of research or reports written by Dr. Kuhn. All such citations are secondary and tertiary sources that merely claim the research and findings were done and reported by Dr. Kuhn, all quoting an article on the miraculous nature of the image that claimed the research was done by Dr. Kuhnwithout attributionin 1992, or other articles referring to the same 1992 unattributed source. This is not even good scholarship much less science.
This is the current state of "scientific" research on the image of the Virgin of Guadalupe. Most of the scientists who have examined the image have done som through the rose colored glasses of faith. Even some of the cursory examinations, even performed by scientists of Faith, have concluded that the image is a work of art. Others do not.
As I said, these scientific examinations are merely cursory. No samples have been taken, no credible chemical testing has been done, no electron microscopy, no spectrometry, or carbon dating has been done. To conclude that the image is not explicable by science is, at this point, premature because science really has not attempted to do so and no comprehensive scientific examination has been done.
It is possible that the image on the tilma was a real miraculous apparition, but I believe that if so, it has been "enhanced" over the years by pious artists who thought they could "improve" the image. Certainly, the symbolic images below the image's feet were probably added, as well as the gold enhanced aura around the image. What is almost miraculous is that the Shroud of Turin, given the examples of other enhanced and illuminated relics, and the tendencies of the time, survived without such artistic "improvements."