"The origins of memento mori photographs can be traced back nearly to the beginning of photography itself. During the nineteenth century, post-mortem portraits were used to acknowledge and mourn the death of a loved one, especially a baby or child. All social classes engaged in the practice, which became more widespread after the introduction of the daguerrotype in 1839. The subjects of the photos were generally arranged to appear as if peacefully asleep, all their earthly suffering ended. Displayed prominently in the household alongside other family photographs, the portraits helped heal grieving hearts by preserving some trace of the deceased.
For a wonderful site on the subject go HERE