How to remove thin grey border around images

Hi all,

I am working on a new WP site and am using the free Path theme. A thin grey border is showing up around all of my images and I would like to remove it. If anyone could help me identify the issue and how to resolve it, I would very much appreciate it.

I’m a bit of a novice with the CSS code, so if you could imagine you were telling your Grandma how to do it, that would be great.

Example: https://istudyenglishonline.com/english-vocabulary/basic-beginner/pets-animals/

Thanks!