I use MediaMonkey occasionally. The catch with it, for me, is that it will show album art for a tune if it's embedded or present as a folder.jpg, and I can't tell from within MediaMonkey which type of album art it is.
Anyway, I used Mp3Tag to look at the test files and there is only one embedded album art image in each mp3.
I'm really surprised the "black box album art" issue hasn't crossed your path.
Do you use MCE 2005?
Do you have IE7 installed on the MCE 2005 PC?
There are tons of forum posts about this issue, many of which say that by rolling back to IE6 the prob goes away, but I really like IE7 and don't want to roll back.
My findings so far are that if the file has an embedded image, MCE 2005 displays a black box. Delete the image and MCE will use folder.jpg if it exists.
The catch there is that I recently bought a Zune 2 and it uses the embedded album cover art. I'll have to experiment to see if can use the folder.jpg instead.
Thanks for responding. If you have any suggestion I'll welcome them!