If you want to add an custom log to Roundcube:
Addd to /etc/roundcube/config.inc.php
/*
[
// show the image /images/logo_login_small.png for the Login screen in the Elastic skin on small screens
"elastic:login[small]" => "/images/logo_login_small.png",
// show the image /images/logo_login.png for the Login screen in the Elastic skin
"elastic:login" => "/images/logo_login.png",
// show the image /images/logo_small.png in the Elastic skin
"elastic:*[small]" => "/images/logo_small.png",
// show the image /images/larry.png in the Larry skin
"larry:*" => "/images/larry.png",
// show the image /images/logo_login.png on the login template in all skins
"login" => "/images/logo_login.png",
// show the image /images/logo_print.png for all print type logos in all skins
"[print]" => "/images/logo_print.png",
];
*/
$config['skin_logo'] = null;
And make sure you use a relative path from /var/lib/roundcube/
Or use a valid url