A detailed attempt at understanding the moment to illustrate -
"Pulp Fiction covers always seem to be trying to cram as much action into the cover as they can (a lot of comic book covers seem to try this approach too). For example, someone's breaking down a door right as they're shooting another person who is just at that moment burning a screaming person with a white hot poker."
The other school of thought I like to think of as more of an "Illustration" approach. Painters like Pyle and N.C. Wyeth seemed to have a different method of finding the right scene to portray.
These types of paintings seem to be created by showing a moment either before something momentous happens or right after something momentous happens. I like to think of it as they show either preparation or aftermath. But not the moment of action.
In this Wyeth illustration from "Treasure Island", the heroes are preparing to raise the flag of their fort, in defiance of the pirates that are laying siege outside the walls. He could have painted the moment where the flag is raised in a heroic fashion against the sun, with pirates firing their muskets at it and the heroes straining to raise the flag. It could have been painted as a big dramatic moment, but instead Wyeth chose the moment before: the simple action of the character sewing the flag to the flagpole and preparing to hoist it above the fort.