They were absolutely capable of painting a negative of a portrait.
And, by the way, the image on the shroud is not made of paint, so contemporary proficiency with painting techniques hardly seems relevant.
In the same vein, why would it have to be made of paint? Paint is simply pigment inside a medium. Dyes are also pigment, in different medium, made to soak into and bind with cloth instead of being layered on top.
I'm not saying that is how it was created, but I highly doubt that the skills to do so did not exist.
In my view, it is a big problem for the fraud hypothesis because you have to explain why and how it was done. At a time when the idea of a photo negative was entirely unknown, and when there are no other examples of negative images, or even any mention of the idea of making such images, why would the fraudsters seize on the idea of making their fraudulent image a negative? There is no record of anybody even recognising that it is a negative until the 19th century. So, it’s not at all what you would expect given the fraud hypothesis. You would expect a straightforward image.
It was not called a negative until the 19th century when photography came about, because before that a photo negative wasn't a thing. Before that it was just a "painting with light and dark reversed".
> when there are no other examples of negative images, or even any mention of the idea of making such images
There are many examples. See woodcuts, for example. The concept of creating the negative of an image was common.
> why would the fraudsters seize on the idea of making their fraudulent image a negative?
Because they, and the intended consumers of their piece, were not stupid and all were aware of the pattern that a person would leave on a cloth. Presumably it was not more difficult to drape a cloth over a body and observe the staining pattern then than it is today.
> You would expect a straightforward image.
No, you would expect a straightforward image, because photographs weren't invented until many centuries later, apparently.
If you put a piece of cloth on your face, the parts of the face that touch the cloth are the ones where color would be transferred from your face to the cloth, and which are therefore darker. The parts of the face that are more recessed, like the areas around the eyeballs, would not touch the cloth, and so less or no color would be transferred from the face to the cloth in those areas.
In other words, the part of the face that would receive more shadow in a normal image (and would be darker) would receive less pigment in the painting (and would be lighter).
Why is it hard to imagine the perpetrators just painted a model with red ochre and draped a cloth over him?
And you are misunderstanding the burden of proof, fraud is merely one possible explanation. Even if you show that its real blood instead of paint, how do you demonstrate that its not the impression of a dead medieval knight and was later mis-interpreted as a shroud of Jesus after the original creation was forgotten?
Those who would claim its actually the shroud of Jesus have the burden of proving it could possibly be old enough, that it could have been woven by peoples of that time, that the red ochre is actually blood, that Jesus actually existed, that he was crucified, and that he was laid in this shroud and its his blood.
And even then that doesn't show he was resurrected, or that he was divine, or a god, or anything to that effect. We have a ton of evidence Jesus wasn't, his own failed prophecy of the kingdom of heaven coming before the disciples passed, and his crying out for god on the cross.
Mostly we have the fact that the super active, super public god of the bible disappeared after all these stories were written and hasn't been seen since, despite claiming its the most important thing in the world that people know he exists. Essentially damning more than 90% of all the people who have ever lived to hell.
You're calling it a photo negative. It's not a photo negative. It's a painting that has shadows and lights inverted.
"the image on the shroud is not made of paint"
It's made of red paint.
This has been conclusively, scientifically debunked multiple times.
There are plenty of examples of engravings, carvings, intaglia, and so on that used what we consider to be a "negative." There's nothing particularly special about flipping an image, transposing light and dark, inverting the 3d characteristics, or otherwise reversing different aspects.
Specifically, the inverse image might be carved for a wax seal ring or imprint, or it might be carved for a decoration stamp used in cement, or a mold for jewelry or ceramics. There are plenty of examples of things all throughout history that provide opportunity to inspire an inverted or "negative" image; it's simply our context of photography that is novel.
If you want to get Bayesian on it, the base rate of confirmed art forgeries and religious artifact forgeries is non-trivial, but the base rate of confirmed creators of the universe manifesting has human form is zero.
Strobe lights didn't exist in the 14th century either.
Also, is the implication that undead Jesus was twitching rapidly and flashing on and off? That is kind of cool, perhaps I should get religion.
I don't see that as a reason to Revere any item.
Point being, people are way too eager to assume supernatural causes.