Featured Boxes with blury images

So I use featured boxes on my landing page. Now I think it would look great, except that the picture size (chosen by the theme) is to small for the box which makes it look totally blury.

When I investigate with ‘inspect element’ I can see this when I hover over the image: “297 × 238 pixels (Natural: 212 × 170 pixels)”. The strange thing is, I configured the box to have a height of 170 pixels in the customizer. So maybe this is a bug instead and is it these customizer that doen’t actually make the box 170 pixels but rather 238 pixels high? Or is there, for now, a way to force the theme to choose a bigger version of my photo?

Thanks a lot!

Website: noplacelikeoutside.be