I recently uploaded a short video and upon completion there was no thumbnail. I checked the database entry and the link to the thumbnail. There is no way to change this in administration.
Database:
Directory Contents:
Here is the image name that ES is looking for:
/media/com_easysocial/videos/1/5efebedb83182fb390b39e99811e5a63.jpg
Image URL
There is no .jpg file in the directory.
Note - I have self-resolved this issue and disabled video upload temporarily.