Postmortem photography usually refers to something from the Victorian Era, but it is re-appearing in a different way. The postmortem photograph was used to have a lasting keepsake/image of the deceased. Nowadays, it is more common to see people taking photos at funerals, and having them appear on the internet, unfortunately often on gore sites. Now, because photographs are so easy to take, it would seem creepy to the average person to take photos of the dead, lying in their casket, when you can remember them in such better ways.
I guess I don't see the difference. For me, the need for the last image would still be there, regardless of the photographs I have of the person throughout their life.