For episodes in TV series, both a vertical & a horizontal primary image are needed. The horizontal image is needed in the episode listing, and the vertical is needed on the episode title page. I would love to see an option added to select a primary image for each orientation.
Yes, for the time being that is what we are supposed to do. Even so, I want to put this in as a suggestion for feature enhancements.
For the shows that would not benefit, they could continue to use the current method. For those shows that would benefit, contributors could select a vertical primary as well. Some shows have created vertical posters for each episode.