Symbolism, Imagery, Allegory
Whenever you have a painting described at length in a novel, you know it's got to mean something. So when we find out that Tulkinghorn has a giant painting on the ceiling of his office of a guy pointing his finger down in an angry way – well, that sounds symbolic to us. Symbolic of what exactly? It's not clear in the novel. On the most basic level, the finger ends up pointing to the spot where Tulkinghorn will die. On a more metaphorical level, it might be a finger of accusation. (Tulkinghorn is one evil dude, so that's a safe guess.) Or maybe it's meant to be an indictment of the system that Tulkinghorn is a part of. But Shmoop's money is on the idea that this is an indirect representation of the third-person narrator.