Surely each cd, no matter how old has the cover artwork in it somewhere?
The cover art for a CD is nowhere to be found on an Audio CD. The audio CD just contains the music in CDDA format (to be played on CD players). Cover art comes from external sources. Many times these are scans of an actual cover. Other times they come from a band official web site or other source.
MC's treatment and handling of artwork is a little bit complex because it can be stored in several places. I'm not 100% sure how the Pono handles artwork, but I do know one thing: Pono can read artwork that is embedded inside FLAC files.
This is the best way to be sure that your cover art and music files stay together: Put the artwork INSIDE the FLAC file! That way they are one and the same. How do you do this? Check this option in MC:
Tools > Options > File Location > Cover Art > Also store image in the file's tag > (check)
This will make all future cover art saves go inside the FLAC files themselves. But I'm guessing you have hundreds or thousands of files that do not have their art stored inside them. You can update those files by highlighting them, then: <right click> Library Tools > Update Tags (from library)
That should embed any existing cover art inside the file. Test this on a single file. Then do a whole album. If that works ok, I would do something like 20 to 30 albums at a time and update them all.
Going forward this should be automatic, as long as you have the option to store art inside the file checked. Note that this will work great with FLACs, but not with some other file types like MP3 and WAV.
Good luck.
Brian.