In case you didn't know, a track entry in Music (and previously, in iTunes) can have more than one artwork image associated with it. In such cases, the first of its artworks is what I call the "display artwork" and the other artwork is what Apple calls...the "Other Artwork":
The "Album Artwork" image on the left is what is displayed throughout Music/iTunes as the artwork for the track. The "Other Artwork" doesn't get displayed anywhere except in the Artwork pane of the Info panel. And "Other Artwork" doesn't always mean other artwork singular; you can really go to town and add thousands if you want (probably; didn't try; crazy idea). Incidentally, I'm pretty sure only one image can be written a track's file metadata and that will be the "Album Artwork" image.
In AppleScript, a track contains artworks and each artwork is accessed by index: artwork 1, artwork 2...artwork 4732. The "display artwork" is always artwork 1.
I am currently preparing an update to the script Re-Apply Downsized Artwork, which, among other things, can insert a new down-scaled image as a track's artwork 1. If the track already has artworks, they are "pushed up" such that the current artwork 1 becomes artwork 2, the current artwork 2 becomes artwork 3, and so on, so that the new image can become the new artwork 1.
Here's the bug-or-feature part of the essay (mostly bug I'm thinking): When you ask for any other existing artwork greater than 1, Music gives up a parameter error. This makes it impossible for AppleScript to access any artwork data other than artwork 1.
I may release the update to Re-Apply Downsized Artwork that side-steps the "insert" feature, but I thought I'd explain why I haven't released it yet at all.