Images not in proportion

I have the Responsive Directory Theme 6.6.2 and version 3.9.2 of wordpress and when a user uploads multiple images the images when you click on the thumbnail are not in proportion, they seem to be squashed.

When you click on the image directly it opens up in full screen mode and displays perfectly.

Any ideas on how I can constrain the images so they display correctly?

https://www.poza.co.za/listing/sport-and-lodge-photographer/ is an example