I have several music albums which do not have official releases on MusicBrainz.
I’ve embedded tags and album cover art in each track on these albums. I’ve moved “local media assets” to the top of my agents list, and made sure these albums remain “unmatched.”
When I click on “posters” in PMS, none of my album covers come up and I have to drag and drop them manually.
The settings in Settings > [Server Name] > Agents only apply to legacy agents. If your library is using Plex Music as its agent, you can configure agent preferences for each library individually.
See: ⋮ > Manage Library > Edit... → Advanced > Album Art (on the library’s Advanced tab you can also verify what agent is set for that library; see: Agent).
Admittedly I haven’t tested this in some time. Where I have a local poster, I keep 1 poster file in each album folder – that way I don’t bloat the individual files
If there is an embedded cover and a separate cover.jpg file, Plex will prefer the separate file (separate files tend to be higher resolution than embedded images).