Thumbnails all one size?

Hello, I am loving your theme, and really appreciating the simplicity, responsiveness and mobile-friendliness of it! It displays images beautifully at the top of posts. Quite stunning, really.

My problem is that the thumbnails on the front (when using excerpts as a post page) and archive pages are all different sizes. This makes wide images teeny detailed (I would prefer them to be cropped) and doesn’t have the uniform, clean look that I prefer and that would really compliment this theme.

Can anyone please direct me to what I can do to enable this, maybe in a child theme (I have a little experience with them)? A link or even just what this is called so I can search it would be great.

It would also be a great option in the theme ??