Image too big on homepage

I’m using the Pinboard theme for a blog.
When I want to add a featured image to my post, it just looks huge on the homepage. Even when the size of the image itself is small.
So I would like to know how I can obtain a post with a featured image with a normal size. Like the exemples on the theme homepage.

Can anyone please please please help me with this?
Oh, I should also say that I’m new at this ??

Thanks!