This was one of the additions of Subdued 2.8.1 – See here
The existing behavior was to show just the episode thumbnail. Something always bothered me visually about it, though. The thumbnail is typically a landscape aspect (vs. the portrait aspect of poster art), and so doesn’t fill the space vertically. So I changed the details view style and the item details page (which you've shown above) to use the season poster and overlay a smaller version of the episode thumbnail. Admittedly, now that I see it with no CoverArt treatment, it doen’t look as good as it does in my setup (shown below). There is desire that I make an option to go back to the old-style, so I may just do that in the next version.