Currently, any thread which is in a forum where thumbnails are enabled, has CSS class 
has-thumbnail. However this does not actually reflect if a particular thread has a thumbnail or not. With this release, we jave added additional css classes 
has-thumbnail--real (thread has a thumbnail based on attachment/media/image found in it), 
has-thumbnail--default (thread has the default thumbnail defined in the admin panel) or 
has-thumbnail--none (no thumbnail is used and thread starter avatar will be shown instead).
The new version is available for all licensed customers at 
https://customers.addonslab.com/marketplace.php/my-services/
Thank you!