Ah! Thank you for your explanation. I think I understand now.
It sounds to me like you are using the Picard application just as a tool for you to type in metadata (song titles, artists, etc.) and drop in cover art. You are not pulling data from the MusicBrainz database at MusicBrainz.org .
I have good news! The MusicBrainz database can make your life much, much easier — especially if your music files are songs which other people enjoy and have described in the database already.
There are instructions for how to do this. Briefly: import a directory of music files into Picard. A number of files may end up in the left pane of Picard, under “Unmatched Files”. Select those files, and click the “Scan” button in Picard. Picard will calculate a “fingerprint” of the song from its audio, and use that fingerprint to look up the song in the MusicBrainz database. If it finds something, then a “CD” icon will appear in the right pane of Picard, and your music files will move from the left pane to the appropriate track of the CD icon in the right pane. At this point, Picard fills in the metadata (track title, artist, etc.) and the cover art automatically, for all the music files it matched! You press the save button, and those files are saved with the improved information. No need to manually add cover art images to each file.
You are left with the music files which Picard was not able to recognise using fingerprints. Here, the more difficult but better path is to learn how to add entries to the MusicBrainz database, add entries to the database for your unmatched files, then match your files to those database entries. This makes life harder for your, but easier for the next person. The easier path is to simply edit the metadata and add cover art, file by file, as you are doing now.
By the way, the Picard quick-start tutorial is helpful, if you haven’t seen it yet.
Good luck! —Jim DeLaHunt