Tumblr theme vertical photos Options

[photog35]

Hi guys,
I am needing some help in adjusting a tumblr theme for my photography blog. (I have no knowledge in html or css).
I would like to make the vertical photos on my blog have a smaller width than the width of the horizontal photos. Right now, the vertical photos have the same width as the horizontal photos (which makes the vertical photos look really big compared to horizontal photos). Is there a way to change that?

Here is the free theme I am using: https://otherthemes.com/themes#centre
(Their theme support isn't available anymore, which is why I am asking for help here).

Thanks for your help!