If you're storing all your cover art in a single directory, you can change the location in MC Options, and then right click on tracks and do a "Quick find".
Tried that first, and it found a few "lost" covers. But hundreds remained unfound; no idea why.
By looking at the Image File field I could see the tracks had the correct file name, but in an old, non-existent cover art folder location. I could have fixed them all immediately by changing the folder portion of the Image File path, if File & Replace could act on this database field.
Instead, I manually reconnected the "lost" images to the tracks by selecting each album, then locating its image the cover art folder. I got through most of them, so my immediate problem is fixed, the hard way.
But "for next time", it seems desirable to be able to edit the Image File field just as any others. Is there a reason why it is "off limits"?
(I suppose Plan C could be to rename the "lost" cover art images so "Quick find" would find them. Is it locked into searching for the same file name pattern specified for naming new covert art?)