I am getting a mixed behaviour , If I set the Cover art for folder and inside the tag in an external editor I get a specific image per disc (the desired effect) , if I drag an image onto the Tag Editor grid , I seem to get One Image per album and then some albums show that image and others show the original one
I can't say for sure, but it's likely that when you drag the image to the Tag Editor grid, MC writes that image into the file header for any current selections, replacing all images that might have been placed there previously with your external editor. You can easily check that.
I find that MC support for multiple images within an audio file header is somewhere between very poor and non-existent. The external editor Mp3tag does a good job of adding multiple images to my audio files. It provides complete freedom to use different images in different albums. MC typically sees only the first embedded image and treats that as the "cover art". I also keep the image files in the same folder as the audio album files. Then MC's View Extras feature (RMB on track or album thumbnail in Content Panel) lists the set of images for display (click to display, then <page down> and <page up> keys cycle through the images).
I carry it one step further by defining
image views which look at my audio folders (all in the same library). Then "sync" the image views with corresponding audio views by using identical display sorts for both. This requires maintaining the same values in whatever tags I sort on (such as [album], [album artist], [date], etc.) in both image and audio files. The net result is that I can get a simultaneous view of all album art for any album.
1/18/2024 Clarify