WMP has a cache for album art.
see %appdatalocal%\Microsoft\Media Player\Grafikcache" and delete the folder LocalMls
Also: set all folder.jpg and albumartsmall.jpg files to visible, non-system:
(best in a command shell: open file explorer, navigate to the top-most folder, Shift-right click for a context menu and select "Open command shell here". In the command shell enter the following:
attrib -h -s /s *.jpg
and delete them:
del /s folder.jpg
del /s albumartsmall.jpg
Treat all thumbs.db in the same way.
Close the command shell)
Then load your files into MP3tag and export the album art with an action to the name
folder.jpg