[Gateway] Images stretched in posts, look fine as features images on homepage

Hi everyone, i have a weird issue.

my features images look fine on the homepage.
https://entertainicus.de/

And I want them to look exactly the same, with the same dimensions in the posts themselves, however there they are stretched for some reason.
For example here https://entertainicus.de/2015/07/15/entertainicus-praesentiert-nerdenegger-reviewt-terminator-genisys/
and here https://entertainicus.de/2015/07/06/folge-2-game-of-thrones-staffel-5/

Help would be greatly appreciated. Thank you.