There’s already an existing suggestion discussing an option to lock posters the same way you can lock other metadata attributes. As you pointed out, Plex will already lock the selection once you change the poster selection (and as long as that poster remains available, e.g. it’s not taken down due to some legal/copyright issue on Plex or one of the metadata repositories used by Plex).
Please comment/vote in that thread in order to help us avoid distracting or cannibalizing votes. Unless I’m missing some unique aspect of your suggestion I’ll close this thread as a duplicate.
