overlapping catalogue images displayed on shop page

Hi,
On our shop page the category images are overlapping. How can we make them display with a margin around them?
All other product images seem to be spaced ok on the site.
The offending page is here:
http://www.blackstackstudio.com/v1/shop/

Not sure how to fix them while not affecting the other images which are displaying fine.
I have noted that by changing the browser size the images can sometimes appear with a margin but only when the browser window is narrow. When a wide screen is used they overlap.
Any ideas?
Thanks,
Joseph.