poster.jpg will always have priority over folder.jpg. Playlists get posters differently to files, folders will use folder.jpg if it is found but active playlists will try to get the poster from the Internet if the settings to get metadata from the internet is enabled, if it is not then the poster will come from the first file if it has a poster. When creating the library the files may not have the poster assigned yet when an active playlist is created so in this case the poster will be set when you browse the playlist.