when a link is shared the is no thumbs
Currently that is no setting to allow admin to set placeholder image for the link preview.
But you can download my attached file and replace into this following file location.
JoomlaFolder/administrator/components/com_easysocial/includes/crawler/hooks/opengraph.php
Then put your placeholder image under this file directory path -> JoomlaFolder/images/placeholder-image.png , the image name and the extension name also have to be the same.
So system will detect if those link page doesn't have provide opengraph data for image, it will use your placeholder image.