Only GW Bush looks solemn in this picture. GHW is too old so I can't tell if his expression reflects his feelings. All the others smile like it's some festive social gathering.
Perhaps it is a somewhat festive occasion. Sometimes, when a person has been ill for a longtime, their blessing is that they are free of all the illness and suffering. My wife wanted me to throw a party when she died. I didn't. I regret that when it came down to it, I caved and went with tradition for the sake her family and friends. Like Barbara Bush, my wife made a decision to stop all medical assistance keeping her alive and extending her pain and suffering. When my time comes, I hope and will be as brave as she was.
It could also be because of tradition that everyone in this photo is smiling. It's what we are expected to do when having our photo taken. Can you imagine a group of people drawn together for a photograph, weeping and mournful appearing. It would be absolutely weird.
To my way of thinking, photographs after services or at memorials are totally inappropriate. Folks are already conflicted enough and shouldn't have to put their emotions on display.
On a much lighter note, Hilary is a shrimp as compared to the rest of the group. Maybe she wore flats.
Just an observation, but Trump's being absent seems specially off to me. All these folks are accustomed to the fervor involved when protective services are required. In my opinion Trump using this as reason to not attend is garbage and definitely in very poor form, although hardly a surprise, considering.