Originally, "memento mori" meant almost anything -- locks of corpse hair woven into jewelry, death masks, paintings of the deceased -- just so long as you were probably haunted for carrying it around, it counted as an MM. The advent of photography changed all this. Suddenly, the middle class could afford to have the pallid, waxy corpses of their loved ones immortalized on a budget. And, since early cameras had exposure times of up to 10 minutes -- meaning the subject would have to sit really still (corpses are generally pretty still, and if they're not, you've got bigger problems than a blurry picture) -- the photographers were generally down to take some soft-core photos of the dead.
The relaxed posture sometimes added a cheery, casual air.
For a truly fashionable corpse display, the bodies needed to appear as lifelike as possible. This was achieved with a number of cheats, not unlike the ones food photographers use to trick you into thinking a Big Mac is edible. A common technique was to prop the stiff's eyes open or paint pupils onto the developed picture (because that's what corpses need help with -- augmenting their unblinking vacant stares). Children were sometimes surrounded with their favorite toys and, for a side of extra creep, occasionally done up like zombified cherubs. For adults, a complicated pose-a-corpse apparatus enabled more complex stances.
Turn the crank to see him perform a jig.