Hi,
I had taken a look at your webpage source code.
There is no image source code for your post thumbnail.
I am not able to determine what’s wrong with your setup.
You can do a plugin conflict check.
1) If you are using a server cache or cache plugin, please clear it’s cache content and disable caching via it’s settings.
2) Proceed to disable one plugin at a time, clear browser history and re-visit your post thumbnail. Check if it’s working properly.
Do this until you find the plugin that’s causing this issue, please replace with an alternative plugin.
If you are still unable to fix your issue, you will have to hire a developer from https://jobs.wordpress.net to login and troubleshoot for you.
Thank you