I am rereading the Phule's Company series by Robert Asprin. I have "Phule's Paradise" in paperback (Feb 1992 printing by Ace) and it has cover art by Robert Grace. The cover art shows a particular scene where a human mob troublemaker is hitting Tusk-anini and Tusk's partner, Super Gnat (working undercover as a waitress), beans the troublemaker with a drink tray. That part of the cover art appears correct but the bar/casino is full of aliens. The only humans are the troublemaker, Super Gnat, Phule, and Beeker. All the bystanders are non-human - reminiscent of the bar scene in Star Wars. Yet the book says that there aren't many non-humans at the casino (when explaining why Tusk and other non-humans could not work undercover). Why the incongruity between the story and the cover art?
Most covers in the sci-fi and fantasy genre have very little to do with the book they are on. They are mostly commissioned by the publisher and editor to appeal to a certain audience. Unless an author is a really big deal, it is unlikely they have in their contract cover approval. Cover approval is the ability to say "No, I don't like that one, lets get another". Most have cover consulting, which means different things in different publishers, but mostly means you could maybe pass some material on to the designer to influence their design.
In the case of Robert Asprin, he was writing pulp for mass market distribution. That means that Ace wasn't looking to spend a ton of money on his cover. I'm sure that they handed a blurb to Robert Grace and said "Paint us something". Grace had done some other commissioned covers for ACE, but he was by no means a prolific artist. Basically, when it comes to covers, they have little to nothing to do with the actual plot of the book. They are designed to catch people's eye so they will pick them and read the back or the first few pages.