That’ll depend on what agent you’re using for your library. With the Plex Movie and Plex TV Series agents, Plex will pull the posters from their own backend – which will again source it from a number of different sources (e.g. TheMovieDb.org, TheTVDb.com).
Thank you, Tom. I’m really only thinking about music. I guess the same instructions apply?
Is there no way Plex can read music files like Flac if I have already embedded the artwork?
Plex should also be able to use embedded covers from music files. Make sure your library is configured accordingly (from the library’s context menu: Manage Library > Edit… → Advanced; enable the use of embedded metadata and local/embedded posters).
Usually it’s better practice to store the album artwork alongside your files in the album folder… this way you don’t end up with redundant data in your files (e.g. 1 local file vs. embedding the same poster in 10+ tracks)
I cannot currently find the related support article but it should be the same as with local media assets for movies and tv shows… e.g. placing it as cover.jpg or poster.jpg inside the album folder. Afterwards you’ll probably need to refresh the album’s metadata.