After the invention of the daguerreotype -the first publicised photographic process- many families sought out a less expensive way to remember those who had fallen: post-mortem photography. In the mid 1800s to early 1900s, many people began using a cheaper way to honour their dead rather than expensive paintings. During that period, there was a high mortality rate among children and infants. Illness, lack of knowledge about the human body and diseases left a great deal of children going into an early grave. Families would often pose together -whether it was children and their dead siblings or entire families- as a way to remember their beloved. This remained very popular for many decades to come, regardless of how strange it was.
None of these are postmortems.
The top left one, the baby (which people believe is dead) is just a baby. It is too young to sit up on its own, so that is why it is kinda propped/slumped in the chair. But if you look closely, you can see it is slightly blurry. And that is because it moved; on account of it BEING ALIVE.
The top right one is of 2 siblings. The one slumped in the chair is bored beyond belief. But they were told to sit still and look at the camera. And so that is what the child did. But they are holding themselves up on their own and they are looking at the camera. THEY ARE ALIVE.
I have been told by my friend (who is an expert on postmortems, as well as having bought thousands of these pictures), that this picture is in fact a fake. The photographer used the same girl and took two pictures, and then blended them together to create one picture (and yes, the Victorians were able to do that. While they were not magicians who could make dead people look alive, they were ingenious when it came to photo editing and manipulation).
HOWEVER, I like to think that this is actually of twins, and they are only pretending to be dead and grieving.
Either way, NONE OF THESE ARE POSTMORTEMS. Also your information is incorrect. The first camera capable of producing a picture was invented in 1826. And the daguerreotype was actually introduced in 1839.
And let me tell you, having your picture taken was actually pretty affordable. It was the camera itself that was expensive. Let me repeat that: PICTURES WERE CHEAP. IT WAS THE CAMERA THAT WAS EXPENSIVE.
And so families were able to have pictures taken no problem. And high infant mortality rate had been around LONG before this era. It was only since the invention of modern medicine that it has dropped.
In conclusion, your info is wrong, and it could have easily been looked up on Wikipedia. So stop spreading false information.