Talking about Death - Ars Moriendi

Post-mortem photography has one precedent in a centuries-old tradition of mourning portraiture in painting. These pictures are not at all like the earlier images of the ars moriendi. They are individual portraits rather than generic characterizations, and they depict their subjects after death rather than before. The earliest examples, made in northern Europe in the sixteenth century, show a recently deceased subject, usually a nun or a clergyman, lying or sitting up in bed. Laypersons, especially children, began to be depicted in the seventeenth century.

The analogy is similar in the case of yet another precedent for postmortem photography, this one quite ancient: the funerary effigy.
Republican Roman nobility displayed sculpted images of deceased family members in their family shrines.
The purpose of the effigy was to convincingly exhibit the character and virtue of the dead man. The sculptor did this by rendering the features of the subject as realistically as possible.
For this reason, the most realistic of these effigies were made of wax.
These of course do not survive, but others carved in stone or marble still exist.
According to ancient sources, the figures were more often shown sitting up than lying down