I think that in general, RPG covers are pretty bad at showing all the nuances of what the game is about. Most often it's just some of the iconic/example characters in a fight with a large enemy and that doesn't tell you anything. If you look at the D&D5 Player's Handbook, you see a character fighting a monster with magic. Yes, D&D5 is very much 'about' that, but the trope is now so pervasive that also the cover of The One Ring shows the iconic characters fighting -- and yes, you will have fights, but TOR is very much not 'about' fighting endless hordes of orcs. In fact, I think the cover for The Darkening of Mirkwood shows the spirit of the game much better: crawling through ruins to find out who is pulling the strings from behind the scenes, and definately not seek a direct confrontation with just your little group to back you up.
One I like is the cover of Star Wars: Age of Rebellion: We immediately recognise Leia and Luke, with a space battle in the background. This immediately evokes Episodes IV to VI, and that's precisely what the game is about. But this is dependent on knowledge of the source material -- so it's kind of cheating.
The cover of Dungeon World does a good job of showing what the game is about. It's not a fight -- there are, in fact, multiple fights going on, and everybody has to use their abilities and wits to survive in the ruins they are exploring.
But for sheer quirkiness, I will have to go with the cover of Ryuutama. It shows the travellers in the center: practical clothing, travel gear, pets, dragon shrine at a signpost in the background. The backdrop is a map, and at the bottom is another pile of travel gear. In the corners are the four seasonal dragons, and in the middle of the edges are the four types of 'world dragons'. It gives so many little details, and there is no action going on whatsoever, which gives you time to take in everything there is to see. And that's what the game is about too: taking the time to travel and see the world.