How to exclude related posts without thumbnail?

Hi, I’m working on a client’s site that has this plugin installed, and would like to know if there’s a way to filter the related posts as to exclude the ones that don’t have a thumbnail available (i.e.: the default thumbnail shows), preferably without having to edit the plugin’s source code (but will do so if necessary).

Thanks in advance.

https://wordpress.org/plugins/wordpress-23-related-posts-plugin/