Change events banner size in Beford template

Hello,

I'm working in the Bedford template and I don't like how it displays the thumbnail image as a banner. I know how to hide the banner in custom CSS, but what I'd like to do is change it's shape. Rather than be the full width, I'd like it to be a rectangle centered in the middle. Any ideas on what CSS I'd need to accomplish that/ if it's possible?

Thanks

Toby