In /var/www/inc there is a file named coverart.php. It is called by player_lib.php.rgrep coverart *
inc/coverart.php: * file: coverart.php
inc/coverart.php:$ch = curl_init(ui_lastFM_coverart($status['currentartist'],$status['currentalbum'],$_SESSION['lastfm_apikey']));
inc/player_lib.php:function ui_lastFM_coverart($artist,$album,$lastfm_apikey) {
But I have never seen a cover in volumio-GUI.
Is this feature not active? Or am I wrong and coverart.php has another job to do?