Aurender Music servers handle cover art in two different ways, which happens to be the way most media software associates cover images with files. 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 (gold disk) as album cover art.
A few different media software programs handle cover art much differently, the most popular of which is iTunes. It is with users migrating from a music library managed with iTunes that this question most commonly occurs. While the idea of tagging artwork for hundreds, if not thousands, of albums to correct for this issue may seem challenging, there are a few programs available that offer automatic scanning of a library and automatic retrieval and tagging of cover art to make things easier. Bliss HQ, dbpoweramp's Perfect Tunes, and Jaikoz are a few of these programs available.
Also, if your files are tagged properly, but are located on 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".