I've found that if cover art files are stored locally, they must be "cover.jpg" rather than "Cover.jpg" For those who have their art files saved as Cover/Folder.jpg/png, artwork files are downloaded. By adding the following to /var/www/app/coverart_ctl.php, at line 118, cover art files can be capitalized:
$local_cover_path[] = $local_cover_root.'/Folder.jpg';
$local_cover_path[] = $local_cover_root.'/Cover.jpg';
$local_cover_path[] = $local_cover_root.'/Folder.png';
$local_cover_path[] = $local_cover_root.'/Cover.png';