Musicbrainz is merely linking to Wikipedia, while Plex is trying to fetch bios from allmusic.com and (if failing) from last.fm
So make sure that all your artists on MB have links to their allmusic.com or at least last.fm artist pages.
It might have actually fetched album art either from embedded art in your files or from sidecar graphics files in the album folder. If you have an “organically grown” collection (who doesn’t), you will have some album folders with such tiny album art thumbnails in them. These files can even be hidden in a regular file browser. See How do I remove an incorrect tv show poster - #3 by OttoKerner
