Obviously a post-mortem ph0tograph (or memento mori) is a picture taken shortly after somebody’s death.
After the daguerrotype was invented, it quickly became popular due to the lower cost. Victorian era mortality rates for children were exceptionally high, so a post-mortem might have been the only picture of a deceased child and the only means of remembering, so sad as it might look it actually meant something to people and was a ritual of great importance. Furthermore, the film negative is believed to be able to capture a person’s soul so in a way it kept them alive forever to everyone who held them dear. After the carte de visite invention, prints of the child could also be sent to others who would have wanted to treasure memories of the dead.
Post-Mortem photography used to be almost a form of art. The earliest ones are a closeup of the face or the whole person but without coffin, you can see some of those if you visit some very old cemeteries all over Europe. The child seems to be sleeping at peace. Another popular position for children was a crib or an armchair along with their favourite toy (which was also believed to hold the essence of the deceased forever), sometimes together with siblings or the mother. Lifelike appearance photographs are frequent as well, child propped up with open eyes, even standing at times. Later on the photographs depicted the child within the coffin, along with flowers and again friends and funeral attendees.
After “snapshot” photography took over the world in the early 20th century, post-mortem photography slowly ceased to exist, surviving only in a very few Eastern countries on rare occasions.