One thing that it is not is a painting. Extensive scientific examination has proven then is no paint or brush-strokes present. The method of imprintation is not known.
One story I read was the body was dusted with iron oxide and the cloth laid on top - but - no matter how carefully you do it you still will move the cloth blurring parts of the contact. The book “Skeleton Key” has a provocative epilog (all points easily refuted.).