featured image crop in post view

hi everyone,

I have a wordpress blog site for a local bowling club. The issue I have is when the blog post is displayed on the homepage (the small link to the full post) it shows the featured image just fine beside it. When you click on the link to view the full blog post, the featured image is set at the top as it should be but it it is horribly cropped. if it is an image of people then there heads and legs are cut off and it looks terribly unprofessional looking.
How do i get the featured image to look as it should at the top of the full post page?

I hope you understand my question,lol.

Many thanks in advance.