Aurender Music servers handle cover art in two different ways. First, the scanner checks the files for embedded artwork. If there is no embedded cover art for the file, then the scanner checks the folder the files are located in for a "folder.jpg","cover.jpg", or "front.jpg"* image associated with the files to indicate a front album cover.
When neither of these criteria are met, the software will display a generic image as album cover art.
Unfortunately, some media software programs such as iTunes handle cover art much differently. Instead of using embedded album artwork tags, iTunes uses "sidecar" files containing album cover art and other metadata. Unfortunately, this data is not relevant or viewable outside of iTunes.
Luckily, there are a few programs that offer automatic scanning of a library and automatic retrieval and tagging of cover art to make things easier. Bliss HQ, dbpoweramp's PerfectTUNES, and Jaikoz are a few of these programs available.
Also note: Content stored on Aurender's internal/local storage will be "scanned" for metadata automatically. But, if your files 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.
*Note that a ".png" extension will be recognized as well as a ".jpg".