One factor is the subject matter. For example, a painting of a haunted house or a creepy figure like a ghost or a menacing monster can be very scary. It plays on our primal fears.
The backstory of the painting can make it scary too. If there are rumors or legends associated with it, like it was painted by a deranged artist or it was found in a place where a horrible event occurred. Say a painting was discovered in an asylum where a lot of cruel experiments took place. The knowledge of its origin adds an extra layer of scariness. It makes people wonder if there is some sort of malevolent energy attached to the painting.
Perhaps the painting depicts a long, dark hallway with no end in sight. The shadows in the painting seem to move on their own, creating an atmosphere of unease. It gives the impression that something sinister lurks just out of sight.
The appearance of the clown in the painting can be very disturbing. The bright colors that are usually associated with clowns can be made to look menacing. For example, a big red nose that in a normal clown is funny, but in a spooky painting can seem like a bloody growth. Also, the eyes of the clown in the painting might be too large or have an evil glint, making it seem as if it's watching you. This gives a sense of being observed which is very scary.
There could be a painting in the hallway that seems normal during the day. But at night, figures start to move within it. Maybe it was painted by a deranged artist who trapped souls in it. As people pass by the hallway at night, they can hear faint whispers coming from the painting, and sometimes, a cold hand seems to reach out from it, making the whole place feel eerie.
A family moved into a new house. On one of the walls in the attic was a very old painting of a family. But there was something off about their expressions. One night, during a thunderstorm, the power went out. The children were scared and huddled together. They noticed that the eyes of the people in the painting seemed to be lit up. The family could swear that the figures in the painting were moving slightly. They removed the painting the very next day and burned it, hoping to get rid of the eerie presence.
The unknown. When there are things that are not fully explained or shown, it scares people. For example, in a haunted house story, if you just hear strange noises but don't see the source clearly, it creates a sense of fear. You start imagining all sorts of terrifying things.
What makes a scary story truly scary is the element of the unknown. When there are things that are left to the imagination, it scares people more. For example, in a story where there's a strange noise in the attic but you don't know what's causing it. It could be anything, and that uncertainty is terrifying.
Details. A painting rich in details can tell a great story. For example, if it's a scene of a market, the detailed goods on the stalls, the different types of people interacting, all these details build up the story. It could be a story about trade, cultural exchange, or daily life in that place.
The elements in the painting. For example, the characters. If there are people in the painting, their expressions, postures, and what they are doing can tell a story. In 'The Scream' by Edvard Munch, the distorted figure with its hands on its face seems to be in a state of extreme distress, which tells a story of some kind of inner turmoil. Also, the setting. A dark and desolate landscape can set a mood for a story of loneliness or despair.
Well, for a painting to tell a story effectively, it needs a coherent composition. That means the placement of objects and figures should guide the viewer's eye and convey a sense of sequence. Also, the artist's choice of theme and the emotions they infuse into the work play a huge role.
Once there was a little girl who moved into an old house. In the attic, she found a clown painting. Every night, she would hear strange giggles. One night, she woke up to see the clown in the painting had moved closer. Its eyes seemed to follow her everywhere. She was so scared that she couldn't move. Eventually, she mustered the courage to run out of the room and never went back to that attic again.