I found out about this tradition about 5 years ago, and got a little obsessed with it, I couldn't understand it. Now I guess I'm desensitized, and I sort of think it's neat when you think of it in terms of the times. They rarely had photos of anyone back then, and with tiny ones it would maybe be the only way for the parents to "remember" them clearly.
The most horrifying montage I've ever seen well there are two, one was an entire family shot to death and laid out like piano keys on top of a quilt and photographed, and the other was a small child that had severely sunken eyes.
I couldn't sleep right for months when I was researching this, and I now present a treat for those that are so inclined- the largets post mortem photo database in the world:
Thanatos.net / The Thanatos Archive / Post Mortem Mourning Photography If you are like me you will savor these, and potentially go through a heavy period where you will view life through a more fragile lens, and ponder thoughts before unthunk. Beware.
The boy in the photo that was taken through his window- I remember this photo distinctly from a website that had backstories to some of them, and in this one what happened was he died of a horribly deadly contagious illness and the photographer would NOT go inside. So they laid him next to a large window, swung it open and CHEESE.