Do you want to add them as some kind of photo gallery or is this about local artwork for posters/backdrops…?
Plex doesn’t currently support adding photos as local extras… but there’s a feature suggestion discussing an option to add image galleries (as videos); edit: found another one focusing on doing the same based on plain images.
As for adding posters/backdrops/… → https://support.plex.tv/articles/200220677-local-media-assets-movies/
