Hello there!
Mp3tag ables users to fetch for album information using tag sources. With luck album art is also a displayed. I find this process tedious and quite redundant resulting in a waste of time. I end up not using it.
My idea is to add some context menu to fetch folder image using %artist% and %album% or any other relevant metadata. At same time the image is dragged it could be automatically renamed to folder.jpg. Here's a mockup:
