A quick search shows this "Feature" has been requested more than a few times, across multiple versions of MC (MC15 discussion last July, for instance). PLEASE make this minor tweak to to MC16 to put the problem to rest.
PROBLEM: Find and Replace can't change field Image File. (It is the database field that stores the cover art file path and name for each track.)
There are times when this field MUST be changed, notable when cover art is stored in a specified folder location AND this location is changed. Over the years I've had to move my MC library for various reasons, such as reorganizing a drive, putting cover art or the entire library on a different drive that has more space, etc. Each time, MC makes this a huge, hairy job because I have to use the MPL workaround (below).
It would be simple and elegant to change the cover art path via Find & Replace. I can do this when I move the media files, but NOT the cover art files, because field Image File is treated as read-only. WHY?
Methods and workarounds are available, but they are not always reliable or practical.
In some cases Quick Find will reattach "lost" images. But this works only in the specific case where the image name exactly matches what MC is looking for. My library is 80+ thousand tracks, and very few have cover art that matches what Quick Find expects. This is because most of the cover art was manually added so has arbitrary file names, and/or the track tags were changed after the cover art was attached, so it no longer matches what Quick Find expects.
A workaround is to export the entire library as an MPL text list, find and replace that text with a suitable editor, the import the MPL back into the MC library. BUT... Finding a suitable editor to process a text file of 80 thousand records is a challenge, the process is very slow to do (hours), and messing around with all this data is extremely subject to a mistake that can trash the entire library. (Make a backup first!) Surely it would be safer to use Find & Replace within MC.
SUGGESTION: Allow Find and Replace allows access to the Image File field to change the cover art folder path in the database. AND/OR, upon seeing a change in Options > File Location > Cover Art, automatically update (after a confirmation prompt) the Image File path in all the records in the library.