Ok, so I extracted the artwork from the m4a's using Mp3tag and I named them %_filename%, and put them all into one folder. So for instance the artwork for 'song x.m4a' is 'song x.jpg' and it's in D:\Album Art with all the other jpg's from the m4a's. Then I did some experimenting with the sql query in Heidi. I tried a few things that returned a syntax error, but finally I got this to run, for whatever it's worth (not much apparently):
UPDATE songs
SET `image` = CONCAT(SUBSTRING_INDEX(SUBSTRING_INDEX(songs.path, '.m4a', 1), "D:\Album Art", -1),".jpg") WHERE songs.song_type = 0;
Looking in Heidi in the data tab under songs, in the image column, I could see that there were now images corresponding to the m4a's, where there used to be something like 'no image.jpg' prior to my running the query. But looking closer, it seemed that at least some, if not all of the images corresponding to my mp3's had been appended with '.mp3' in the jpg file name. In other words, in the image column, where it once would have shown the image for 'song x.mp3' as 'song x.jpg', now it was showing the image as 'song x.mp3.jpg'. Something or other I had done previously seemed to completely break the album art display function in radiodj.
Now no album art images are displaying in radiodj.
I continued to try to peck away at this (like a lot of people right now I have nothing else better to do). I have no idea what I'm doing, but why not experiment? If worse comes to worse I can just delete the database and start again from scratch. It's better than getting COVID-19. Alas I have found no solution, and somewhere along the way a bunch of the m4a images now have '.m4a' appended to the jpg name listed in the image column in Heidi as well. So much for my career as a hacker.

I moved my m4a album art to the folder nestled within the radiodj program folder (C:\radiodj\Album Art), so that folder now contains all of the jpg's for both the mp3's and m4a's, where the image file name for 'song x.mp3' is 'song x.jpg' and the file name for 'song y.m4a' is 'song.y.jpg'. Of course that didn't help the broken album art function, but at least all of the artwork is all in one place and is named similarly.

Clearly I need some help (people have been telling me this for years). What can I do to fix this fine mess I've made? Thanks for your assistance.