This script previously used mshots, a free thumbnail generator by the same company who developed WordPress. Recently they disabled server-side access of their API resulting in 403 error - thumbs not showing up. There are two options to fix this issue.
1.0 Switch to PagePeeker (free): To do this, open includes/Short.class.php then find $thumb_provider="mshots" and change mshots to pagepeeker
2.0 Use a custom API: To do this, open includes/Short.class.php then find $thumb_provider="mshots" and change mshots to the URL of your custom service while replacing the target site URL by @URL@ (e.g. http://SOMESERVICE.com/api?key=123456&url=@URL@)
If either or none of these methods work for you, please open a ticket while clearly mentioning your issue and if necessary provide ftp access to accelerate the process.