post images stretched on mobile view

Hi, This theme is great and very easy to use!.
I have an issue with images inside post.
When shirking the browser or when viewing a post on mobile screen – the images inside the post ( not the featured image) keep their original height and only the width is getting smaller. which means the images are very stretched.

Is this because of the theme or am I doing something wrong and there is a way to configure images to keep their ratio ?
You can see the problem very clearly here:
https://blog.booola.com/blog/2015/05/07/my-top-vegan-escapes-in-tel-aviv/

Thanks!