Collection Posters?

[bitmap]

I'm not aware of the logic, but if you search for images on anything you'll see that Jellyfin generally sorts by rating with the highest first. I would guess that's how images are chosen. The image on the left is not generated by Jellyfin, it was likely the highest rated poster image (or the only poster image) for that collection. You can search for images on that collection and see what pops up. Change it if you like.