Cool, but that’s what makes them be displayed as artworks. Generated thumbnails as @tom80H said are previews of the movie or tv show that are shown when you’re fast forwarding it so you can have an idea where you are in the show.
See an example of it here: [RESOLVED] enable thumbnails when skipping
If you don’t use it, disable it in the library settings and it’s gonna save you a ton of cpu processing.
