Changing Post Featured Images in header

Hi!

I stumbled onto Twenty Fifteen which I think looks really great, but I’ve got a problem with the way it is displaying the featured image.

If you look at my site: https://www.throwinghorns.com you can see that on the portrait images, it is cropping from the top. This means everyone has their heads cut off! If it was cropping from the bottom, this wouldn’t be a problem (just based on the way my photos usually look)

Is there a simple way to do this in the theme, or with some tweaks?

Thanks for any help.