When I create a collection from some videos, I expect the poster functionality to behave similarly to posters for movies:
Sibling image files named like “poster.jpg” in the same directory as the collected videos should appear in the collection poster editor.
Automatically apply the first matching image file named like “poster.jpg” to the collection.
As a workaround, I keep having to download the poster images locally, select the collection, open the poster editing pane, re-upload the poster image, and save. For every collection I create.
There’s actually already an existing suggestion from dokuro asking for an option for Plex to pickup local media assets for collections (e.g. posters). Maybe you can comment/vote in that thread in order to avoid duplicate suggestions.