Goto the following via FTP as root:
/usr/local/centovacast/var/vhosts/username/var/www/covers/
(Replace username with the name of the client)
In that last folder is : nocover.png
Create a new cover and save it as nocover.png and replace the one that is in there. (As a precaution, be sure to save/backup the original nocover.png just in case)