Plex will pick up posters for movies if they’re stored alongside your media.
If you upload, link or drag&drop a poster when editing the movie in Plex, this isn’t stored alongside your media but added to the server’s data directory. Paths are usually rather cryptic.
The same goes for manually uploaded/linked/dropped posters for collections.
You should be able to identify the full path of the poster in the server’s database (that can be a pain) or possibly by using some other app like WebTools-NG’s “ExportTools” module.
