Thumbnail aspect ratio?

[palermog]

Thanks again for making this!

I'm wondering where in the CSS it might be possible to change the aspect ratio of the thumbnails? Ideally I would like to have portrait orientation photos not mask the way that they do and appear landscape, but I'd settle for having everything be a square thumbnail. Is this an easy fix in a single place? I tried changing some background-size: cover;s to background-size: contain;, but it didn't appear to have any effect.