@collectoratad, I read your post again and want to add more info.
As I understand it, your Astell&Kern AK500n is your music server that also maintains your stored CD’s. Access your Astell&Kern AK500n to determine where your stored CDs are located (or maybe located on an external hard drive). You need to find the music folder and albums stored within (this took 5 seconds on my Aurender). After you locate the folder, you look for an individual album and determine WHERE & HOW the album cover art is stored. I hope you can find the file with the JPG extension (for example cover.jpg, folder.jpg). It should have a file name and JPG. Next, locate another album that has no album cover art.
Search using the album name and the word “image” (for example, Enya image). It should be found on Amazon (or something similar). Copy the album image to your desktop. Manually change the album name to the name required on your Astell&Kern AK500n unit (for example, cover.jpg or folder.jpg).
Copy this file to the appropriate location on your Astell&Kern AK500n device. CONFIRM, TWICE, the image was copied correctly and named correctly. Use your iPad, tablet, etc. to access the Astell&Kern AK500n and confirm the album cover art is displayed. Continue to add more album cover art.
Unfortunately, this is a time-consuming process. I followed these instructions to add album cover art to my Aurender server and copied 10 albums cover art images per day until I was finished. I hope this helps. Please keep us posted.