Image Gallery Columns

First – thank you for a great theme – easy to style via the child – except: I have an image gallery on my site at http://www.janeforrester.com/hand-printed-fabric/
It is assigned 3 columns. When viewing on mobile devices I would like the gallery to have 2 columns so the images do not scale down quite as much. I’m assuming I need to add some css in the child for @media (max-width: 640px) and have used Firebug to try and determine what should be changed, not having much success and I’d appreciate it if you could help me solve this. Thanks!