if you have a subfolder with the booklet, Plex will usually pick something from there. I usually see Plex using a scan of the disc or page 1 of the booklet in this case. Filename does not matter, I think it just picks the first file.
Unfortunately, this definitely doesn’t work. that was suggested to me a couple of years ago as a workaround to the totally-broken LMA settings at the time; I’ve since spent almost a full year re-working the ID3 tags to work around the limitations of Plex’s music management, and in the process, saved out all of my album artwork as separate image files. Still no dice.
With Plex pushing Tidal hard (SO hard! 
), I have a feeling that Plex Music support has become (was always…?) the redheaded stepchild for the Plex gang.
