Use a camera obscura with a photoreactive compound. Multiple overlapping exposures explains why one image is larger that the other and why the head on the shroud is out of proportion to the body.
That theory depends on some unknown 14th century genius (at least 100 years before Leonardo) who invented a completely new technique with the sole purpose of duping a credulous medieval population. And then completely abandoning photography after taking one photo. It also ignores the huge number of details within the shroud that no forger could have known about, or would have needed to include, to fool the public - things like pollen from thorn bushes.
The head isn't out of proportion to the body. That is a claim of the skeptics that doesn't hold up to their claims. It is perfectly proportioned according to artists and scientists working in their fields of expertise. The skeptics are not working in their fields of expertise. They are measuring from a water stain, claiming it is the top of the head, not the actual top of the head to get their claimed size of the head. That's also how they get their strange body lengths and claim that the image hinges with no space between the fontal and dorsal images.
There is no residue of a "photo reactive compound" which must be present for the image to be still visible. That is what any photograph is made of. . . the residue of the photo reactive compound. The Shroud has been studied with very sensitive instruments down to a Xray micro-spectrometer and there simply are no such compounds or chemicals present. That device was so sensitive it measured the chemicals residue of the special inert vinyl baggies the samples were carried inside so accurately they could tell what company made the baggies. That is how sensitive that test was. . . and they found NO CHEMICAL RESIDUE that was photo reactive to carry an image.
In addition, it is known for a fact that the image does not exist underneath the blood stains. That means the blood stains existed BEFORE the image was placed. Registration of the image after placing the blood would be extremely difficult if not impossible using the camera obscure technique you postulate.
In addition, all such chemical photo sensitive approaches that have been proposed would fade with exposure to sunlight over time. . . and it is known that the Shroud was exposed to sunlight frequently in the past, far more than would be necessary to fade any such image by those means to complete invisibility. Obviously that has not occurred.
Finally we do KNOW what the image is compose of, and the very thin starch coating on the top most fibers is not in any way photo reactive. This is all what the science has lead us to know. . . not skeptics' opinion and musing based on desperate grasping for an explanation based on nothing but speculation.