I need to qualify this statement. There
is paint in the tilma. The thing about the original image (which
is inexplicable) is that it was actually embellished and even slightly altered. The face especially were (infamously) retouched in stages during the last century, resulting in
Our Lady turning somewhat older, somber and slightly fatter in appearance.
(It’s actually pretty easy to tell many of the painted additions; these are usually the places in the image where you see cracking and fading, such as the golden sunburst rays or the black moon or the tassel.)
As per the 1979 infrared study of the image conducted by Philip Callahan, the Guadalupe image would have probably been simply of Mary in a praying pose. (I show Mary’s figure here in full length, but some people think that it’s possible that the image was originally just a three-quarter portrait than a full-length picture, with the lower third of the body being an addition.) The tilma would have been larger than it is today - the cloth was only later cut to size.
Then, the image was embellished by the addition of certain elements: the dark moon and the tassel, then the angel and lower fold (the so-called ‘Aztec fold’, that fringe of the mantle the angel is holding on), followed by the white background and sunburst (and a crown - which was removed in the 19th-20th century), then the gold trim with dark outline in the mantle and tunic, and finally the stars. The thick paint in these areas had cracked and peeled in numerous places, which is one of the things that distinguishes them from the ‘original’ parts of the image.
(Some writers argue that the sunburst and the moon were already part of the original image, albeit the rays would have been more subtle and the moon more smaller - and probably not black.)
There was even a time when someone tried to paint cherubs on the background, but these were eventually erased after they deteriorated badly.