Resizing Images in a gallery template (Wells)

Site URL: https://www.martinvallis.com/

Hello

This is my first post to the forum, hoping someone can kindly give me some advice:)

I'm using the Wells gallery template for my photography site and while I'm happy with the image size, when expanded and in Portrait format. 

However, I'd like to reduce the size of the image when it's expanded and in Landscape format, is this possible?

Many thanks for any help offered.