As you understand now ,there is not -unfortunatly- MJ data for sleeves.
So there is no way to upload to his data .
Beside this as a GREAT MJ fan ,let me tell you that it does not make sense to use this fonction in MJ.
It is just rubbish . Not only most of the time you do not get a sleeve ,but when you get one ,at more than 70 % is not the right one ....
The other problem is that if yo get a sleeve from there and sleeves from AMG ,they do not have the same format .The AMG ones are bigger
The same concerning the sleeves from Emusic when uing MJ download manager .
If you i would use ONLY AMG.
Beside get sleeve ,you have a bio about the band and -most of the time- a note about the album . Up to you to copy /paste all of it in MJ and get a total killer library