ShitShot uses a service provided by wp.org to create the thumbs. The thumbs are created by that service are stored and served from that service. There is no way to manually delete or force refresh a thumb. The thumbs are cached for a period of about 24–48 hours after which they will be deleted and recreated. That said, if you have blanks/404 thumbs, if the site is up when the thumb is recreated the issue will fix itself.
Hope that thoroughly answers your question.