So knowing what the residue consists of (say, X), and knowing what the cloth consists of (say Y), whatever would be not X or not Y would be what makes the image.
Is that not a known substance? Or is there nothing that is not X or not Y, but we see that the image does exist so it is a huge mystery? Or is it that there is so much "there" because of handling the cloth, displaying it to the elements, etc., that it cannot be "boiled down" to just what makes the image?
If you know...
A clear polysaccharide residue coats the outermost fibers of the cloth. In places, that residue has changed to a caramel-like substance through a Maillard reaction. Cooking is a Maillard reaction. That brown substance forms the images.
One possible modality of formation of the image has to do with the gases that are released from a newly dead body before the liquid putrefaction products start destroying organic substances around the body. These gases are extremely reactive chemically. Within a few hours, in an environment such as a tomb, a body starts to produce heavier amines in its tissues such as putrescine (1,4-diaminobutane), and cadaverine (1,5-diaminopentane). These exuded gases react with the polysaccharide layer on the cloth in a Maillard reaction and, in fact, does produce the color we see in the carbohydrate layer on the cloth. However, there is a problem. Whatever created the image on the Shroud acted only in a vertically collimated waywithin a very few degrees of strictureboth up and down with no horizontal component. Gases simply do not act like that.
The polysaccharide coating is fragile and can be broken off the fibers... so the more the Shroud is handled, the more of these coatings will be shattered.