The Church: Formatting blog post images

Hi again. Still happy with our new website. But I notice that the blog posts' featured image gets distorted, please look at this page: http://mernsogn.dk/arrangementer-og-arkiv/ - when you click on "Læs mere" you will see the featured image as it truly should be. Only the candle cross works okay because it's in portrait format. I don't want to use only portrait format because that doesn't work so well on the front page (see www.mernsogn.dk towards the bottom). How can I use featured images in landscape format without them getting distored in this list of blog posts? Thank you, Claudia