I checked your installation – from the backend it looks the featured images should work without issues since the images are being shown on the backend without issues. Unfortunately I lack permissions to make changes to the installation – like no access to theme options. Can you pls provide the same? Also, pls try two things to see if that helps –
1) In the General tab of theme options, pls change the Thumbnail Generation Method option to “WordPress” instead of Aqua and try other way too.
2) Disable all plugins once and check if the problem goes away.