Featured Images no longer loading

I had version 3.8 and I noticed yesterday that all of my featured image thumbnails (for videos, uploaded pdf files, etc.) would no longer load. I tried reuploading and reassigning some featured images without luck.

I upgraded to 4.0 and the problem remains. It happens on Chrome, Firefox, and IE (and I assume others). I diagnosed using Chrome and got the following errors:

Failed to load resource: the server responded with a status of 400 (Bad Request) …/themes/u-design/scripts/timthumb.php?src=&w=220&h=140&zc=1&q=90

Failed to load resource: net::ERR_CACHE_MISS

I’m a WordPress novice, so I hope that diagnosis makes sense to somebody out there! Also, interestingly, the featured images do load when I am logged into the WordPress editor and open the website from the editor. But they do not work under any other circumstance.