Changing Site origin Carousel thumbnail

Hello Site origin Team,
I am having doubts and troubles on how to change the thumbnail sizes to my carousel. My images are squared, but they seem to get squashed/distorted into a rectangular format.

The carousel is on this page: http://divephiphiisland.com/open-water-silver/
Is there any CSS I can use to correct this?
I hope you haven’t answered this somewhere else.

Thanks in advance.

Miguel