Centering thumbnails on category page

I am trying to center the thumbnails that are displayed under each category. These are the thumbnails that you would click on to take you to each individual product page. Can someone please provide insight as to how to get the images centered rather than left justified above each product title? I have the thumbnail size set at 150 x 220 and some of the pictures I would like to use are landscape orientation and others are portrait.

Here is a link to my temporary catalogue, and as you can see certain images are centered and others are left justified. I would prefer to have them all centered: http://beta.ahpd.ca/catalogue/

Thank you in advance.

https://wordpress.org/plugins/wp-catalogue/