Let's get back to a more significant issue: the superstition surrounding this idol. This superstition demonstrates the fact that the image allegedly of Mary is an idol. One example of the gross superstition surrounding this idol is seen in the following paragraph from an apparently popular website devoted to "Our Lady of Guadalupe":
After complying to the Bishop's request for a sign, She also left for us an image of herself imprinted miraculously on the native's tilma, a poor quality cactus-cloth, which should have deteriorated in 20 years but shows no sign of decay 477 years later and still defies all scientific explanations of its origin.(source)
If you look at the image provided on that web site, you might be impressed by the apparent high quality of the preservation (link to image). It looks great. The problem is, that's not the actual image. The actual image has undergone deterioration over the years.
Here (at this link) is an attested faithful reproduction of the image (although one should note that the horizontal mid-line of the reproduction does not appear to have any correspondence in the original). As you can see from the reproduction, there is some real loss of quality in the image. First, there is the most noticeable deterioration along the center of the image. This is due to the fact that the image was painted on a medium made up of two parts, joined along that center line. That's a very natural place for the image to undergo deterioration and damage, just as the edges would be if the image were painted to the edges.
Next, two significant horizontal lines are visible passing just under Mary's hands, and then a second symmetrical pair of lines running approximately through where her knees would be. Given the symmetrical nature of the damage, one is inclined to attribute this to damage from folding the image at some point in the past.
Further, in the upper right corner there is a large stain that appears to be some kind of water damage. A more severe stain appears in the extreme bottom right of the image.
Moving from the blank space of the image inward, consider the outer edge of the corona that surrounds Mary, just beyond the rays that extend from her. The quality of this corona seems to be seriously degraded, particularly at the points where the horizontal fold lines pass through it, but also at other points along the edge of this ring.
Moving inward again, and considering the rays, we see that there is serious loss of paint in many of the rays. This is more pronounced on the right side of the image, although the the sixth ray above the mid-line of the image on the left side appears to have lost paint along about 60% of its length.
We similar asymmetrical loss of paint in the crescent moon under Mary's feet. The right hand side of the moon is slightly more deteriorated.
Moving still further inward, consider Mary's cloak. This cloak originally was covered with stars. These stars appear to have either oxidized or faded in an uneven way such that some of them are quite dark whereas others appear to have a brighter appearance.
Hopefully the above serve to illustrate the general point. Now, of course, some of those problems identified above may only be germane to the reproduction, so it is worthwhile pointing out another reproduction (link). This reproduction shows less deterioration, but some of the same major elements, including the stain in the upper right (less pronounced), the vertical line through the painting center, the horizontal lines (less pronounced), and the loss of paint in the rays and moon. The corona looks much better in this reproduction, but there is still deterioration visible.
High quality photos of the original at the present time seem to be hard to find. Many are from a bad angle (example) or seem to have some focus issues (example) or exposure issues (example). Still, these photographs confirm that the main areas of deterioration identified above are in the painting that is being displayed as the image of "Our Lady of Guadalupe."
So, what is left of the claim of miraculous preservation? Well, someone might claim that this is pretty good preservation for a nearly 500 year old painting, especially considering what it has seen over the years (allegedly, a bomb was exploded nearby the image on November 14, 1921. However, since 1993, the image joined the pope in being protected by bullet-proof glass, presumably out of a concern for the idol's safety just as the pope's safety became a concern). Nevertheless, the plain fact is that while there is still a visible image, it has deteriorated over the years. The claims to the contrary are just superstition - superstition that is contradicted by the photographic evidence.
UPDATE: Let me anticipate a quibble. The quibble is that the paint may have decayed, but the medium on which it is painted has not. It should be obvious that the superstition is about the image as a whole, but in case the quotation above wasn't clear enough:
"The colors have not faded and the cloth has not deteriorated." (source)
"The tilma remains just as vibrant as ever, having never faded." (source)
"The image of Mary emblazoned on Juan Diego's tilma has not faded in nearly five hundred years, nor has the fragile ayate fabric deteriorated ... ." (source)
"There were no signs of cracking or fading on the original image." (claiming as well that "The face, robe, and mantle of the Virgin are declared inexplicable by science. The sunburst, tassels, cuff, moon, gold border of the mantle, stars, and brooch were touched up with paint by Fray Miguel Sanchez trying to enhance the image. Those overlays are showing significant cracking while the original is in perfect condition.") (source)
"It is remarkable that after more than four centuries there is no fading or cracking of the original figure on any portion of the agave tilma, which—being unsized—should have deteriorated centuries ago." (makes similar, though more nuanced, claim regarding human additions to the image) (source)