*bump*
The wiki page cited above does not answer the question, which is whether there is a simple way to have MC embed the cover art into existing files. I suspect I'm in the same position as the OP, with thousands of mp3's which have the cover art in accompanying folder.jpg files and a new device which requires the cover art to be embedded in the files themselves. External cover art files also get left behind if I use MC's Rename, Move & Copy to rearrange files on my server or move them to a new server.
The only way I've found to force MC to embed the cover art into existing files is to use the Get Cover Art tool to firstly save my current cover art to the server, then Remove Cover Art, and finally Get Cover Art again to pull it back from the server and embed it into the files. This is tedious in the extreme, and it would be so much simpler if I could simply right-click and select something like "Cover Art --> Embed in file". This would seem to be such a basic and simple request that I'm surprised it's not already available - or am I missing something?