main image on homepage is always cropped

Hi. I have tried searching for an answer to this question but i’m afraid I don’t understand any of them! Basically, my main image on the homepage (ie. The featured image of my latest post) is always cropped and missing a lot of the image. I don’t know code and am a total newcomer, so if you could help by explaining it simply and clearly as if I am a total idiot (maybe I am) I would be really grateful. Many thanks in advance. https://commutingbybicycle.com