Site image cropping

Site URL: https://www.insearchofgames.com/

I'm working on a site, adjusting the layout - I've designed a banner image to display above the homepage content. When I upload the image though, it always crops too tightly and no matter what size I crop the banner down to myself it never seems to work properly. Hoping someone can help me understand how Squarespace formats images when they're uploaded so I can make this work. I'm using the Ethan template (7.0)