Blog post thumbnails disappearing

Hi

Since upgrading to 3.5 I have noticed something really strange. Featured images (post thumbnails) have randomly started to disappear from blog posts. When I go to the post edit screen, the image shows in featured image section. When I go into new media manager, the thumbnail also shows there too, but when I click to edit image, it says that there is no image data even though it shows the thumbnail. If I follow the link URL for the image on the actual webpage, the image shows up at the link URL too!

Does anybody know what I can do. I have deleted and re-uploaded images for these posts, but some just randomly disappear again.

WordPress 3.5 with WP E-Commerce plugin, W3 Total Cache installed.

URL: https://www.higherstakesrecordings.com/events