Album artwork not displayed on n-Serve from iTunes files on NAS. Solutions?

I have searched the archives and haven't found a solution to this issue.  At least not one that I understand....

I have a Unitiserve connected to nDAC and then to the rest of the system.  The Unitiserve is also networked to (and backed up on) a Synology NAS.  The NAS and Unitiserve are playing well with one another EXCEPT the album artwork on iTunes files which are stored on the NAS (and played via the N-serve app, either the desktop client on my Macbook Pro or the iOS app) doesn't show up.  The art from albums which were ripped on the Unitiserve works fine.  But I have set up a network share folder on the NAS and copied my purchased iTunes MP3s (yeah, I know, but there are only a few of them) in this folder.  They will play fine, but no art.  In the desktop client I can right click on albums ripped onto the unitserve and have an option to edit metadata which would allow me to change or add artwork; however, when I right click on one of the iTunes albums on the NAS the "edit metadata" option is grayed out.

So, does anybody have a way to get the art from the iTunes purchased songs and albums which reside on my NAS to show up on the n-Serve?  BTW, the art from these files does show up when I open them in iTunes, so I know the art is there somewhere.

And, on what I suspect is a related issue, a couple of the HD downloaded albums I've purchased from HDTracks.com also will not display their art, though most will.  These are also stored on the NAS in another network share folder.  The art file is there as a .pdf in the folder with the audio tracks, and it will display if I open it when navigating within the NAS on my Macbook, but n-Serve won't display them.

Thanks.