Confucius, Mencius and Laozi taste vinegar from the same pot. Their perception of its quality is determined by their philosophy, and shown in their facial expressions: One sour, one bitter, one sweet. So it is with life -- even in the same situation, different people will react differently based on their outlook on life.
To me, the painting is a reminder not to fill myself with bitter or angry thoughts. There are many things wrong with the world, but these can simply be stored as facts, rather than dwelled upon and thereby passed on to others in anger.
Consider the story of the vinegar pot as an allegory for social media, perhaps.
I've heard one of the reasons you don't often see people smiling on old photos is because you had to hold the pose for quite a while. So in order to make it easier (and prolly not look like an idiot) they often went for the most mundane expressions.
Not sure if it's true though. Someone more informed is welcome to enlighten me.
For something that isn't too obvious (e.g. not hanged in a museum or anything), I often come back to that picture and it always move me, for some reason :
This image I have of the Beartooth Mountains in southern Montana. On the far right of the image is Paradise Valley, and in the distance on the right, you can just barely start make out Yellowstone National Park.
I don't like picking a favorite piece of art, but I am a big fan of "Eton schoolboy jumping a motorbike". It contains a lot of concepts that I love: Youth, rebellion, freedom, excitement, the UK, the sixties, fashion, and classic motorcycles.
It has this sort of otherworldly serenity. It's dark and scary, but at the same time it feels inviting. It really captures a nuanced understanding of death that I find beautiful.