Open your browser console in Plex Web, and run this command.
Replace <librarySectionID> with the correct ID for your library.
fetch('http://localhost:32400/library/sections/<librarySectionID>/all?type=1&thumb.locked=1&X-Plex-Token=' + localStorage.getItem('myPlexAccessToken'), {method: 'PUT'})
You can also change the type=# to select movie/tv/season posters depending on the library ID used.
- Movies -
type=1 - TV Shows -
type=2 - Seasons -
type=3
Or if you want to do it using Python.
from plexapi.server import PlexServer
plex = PlexServer('http://localhost:32400', token='XXXXXXXXXXXXXXXXXXXX')
plex.library.section("Movies").lockAllField('thumb') # Movies library - movie posters
plex.library.section("TV Shows").lockAllField('thumb') # TV Shows library - show posters
plex.library.section("TV Shows").lockAllField('thumb', libtype='season') # TV Shows library - season posters
