One Image Resizing Differently & Causing Issues

I am currently working on a website for a summer internship for a small company. They have had a website done previously for another business within the company and want the theme to be similar/the same.
I am currently doing a test website using the same layout at the old website and both websites are having the same problem. There are three Jigsaw pieces on the homepage and for some reason the third on is a different size to the other two, even though it hasnt been changed. I have no idea how to change the element.style or override it. If anyone could help I would be so happy! the website is currently being hosted here –> http://www.omgkatrina.com/work

Thanks in advance!