images compressed horizontally?

Just installed this plugin along with a new theme. I am getting images displaying in the lightbox that are compressed horizontally (landscape images appear squished). Any ideas what might cause that?

example:

http://heavyartillery.com/project/test/
(have a look at the two horizontal images in the lightbox, you can see it most clearly there, but it affects all shots – and much worse on a phone than on a laptop screen)

Thanks!

https://wordpress.org/plugins/photoswipe-masonry/