Aurender Music servers are designed to read metadata tags for music files. If album art isn't appearing for your particular album, the files either are not tagged with embedded album art, or the folder the files are located in does not have a "folder.jpg" or "cover.jpg" image associated with the files.
Also, if your files are tagged properly, but are located on an external USB drive or NAS device, they will need to be scanned with AMM (Aurender Media Manager) to be visible, or considered an extension of the library located on the internal storage.
There are many metadata tagging software options available, and many have free trials. We recommend trying a few and finding the one you are most comfortable with using.