https://www.tvmaze.com/faq/42/show-main-images
Policy does not specifically allow or bar the temporary use of movie posters though it does say book covers can be used as a last resort.
I'd like to suggest movie posters be added as a source of temporary images. Further, if they can not be added as a temp source, then they be specifically noted as unacceptable.
As always, txs for reading.
(Note to self for future follow-up: https://www.tvmaze.com/shows/46268/the-mighty-ducks)