Promo art is like any other form of art. Its about the story you're trying to tell. For game promo art, you want people to be able to look at it and instantly understand what the game is about. If the game is heavy on the action, it could be dynamic and "busy". If its a calmer game, something more static can work. There are a lot of options, you just have to be creative with it.
I'll assume that its for the souls-type game in your devlog, right? so if thats the case, I assume that theres action with a grim feel to it. Generally, a grim feel has a darker color composition so you would want to be spot on with the lighting and highlighting so people's eyes are drawn to the point of interest (ie: the character pose, or maybe a character sword swinging and colliding with somethings face)
There are a whole bunch of write ups out there explained by better artists than me on the subject. I suggest reading up on composition theory
Thanks for the reply Calooch! The thing is it is pretty confusing for me to decide what resolution/ratio to use, but I guess that probably means I need to do different art for cover and promotional art..