Lo he conseguido modificando el htaccess para evitar el hotlinking:
RewriteEngine on
# RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http(s)?://(www\.)?xxx.com[NC]
RewriteCond %{HTTP_REFERER} !^http(s)?://(www\.)?google.com [NC]
RewriteRule \.(jpg|jpeg|png|gif)$
http://xxx.com/logo.jpg [NC,R,L]