Thumbnail Sizing issue after WPMU update

We are having an issue on the site after updating Wordpress or WPMU, we think. We have a page that uses the posts featured image as a parallax background image here (full width thumbnails essentially) on this page: . The images are currently hidden with CSS here.

You can see in the screenshot above that the image seems to be pulled and then distorted by some sort of dynamic resizing going on. It never did this before. What can we do to remedy this? We were thinking about resizing the post thumbnail in the functions.php file but didnt want to break anything.

Can you help?