I’ve never understood why this isn’t a “no-brainer” to Plex. You can get the season poster if you add multiple episodes to a season, so the ability is obviously there. Plex shows the Season number and episode in the text portion under the poster, so it’s apparent Plex knows what episode and season the episode is from. Showing the season poster shouldn’t require hours and hours of coding to get that result.
I have several shows that a generic “catch-all” show poster just doesn’t make sense. American Horror Story, Fargo, True Detective, Channel Zero come to mind right away.
Maybe with all of the “Improvements” they’re doing with On Deck, Continue Watching & Recently Added they’ll see that using season posters is just a logical, common sense thing to do. But don’t hold your breath.