No thumbnails showing can be caused by issues at shrink the web, check the log files there and make sure you have the correct settings.

• API keys and then put in the joomla component
• Added the allowed ip / domains
• If on a shared server, do you have the access to use the server's IP

Links

• Today images are not showing | shrinktheweb
• ShrinkTheWeb Modify Account Security, Distributed API, and Allowed Referrer Settings - shared ip addresses are not supported, but not if you are the first to reserve the IP of the shared server.