FUnky image

Hi resepan, the reason for the stretched images is due to your themes css. If you check it on default theme like Twenty Sixteen, it appears fine as in screenshot http://prnt.sc/cv9j4f.

So, I suggest to fix your themes css for this particular case. And the plugin ony provides an easy way to mark a post or custom posts as a featured post. The look and feel is up to the theme designer. For more, you can check @ https://wordpress.org/plugins/featured-posts-pro/other_notes/

All the best