Good news for everybody who’s been waiting for this feature…
Plex Web 4.62.1 supports changing the poster of playlists.
Keep in mind… in order to use this feature you need to use the hosted version of Plex Web (https://app.plex.tv/desktop) until the new version is also included in Plex for Mac/Windows and the bundled alongside your PMS.
Thank you Plex!!! 
