Featured Images for Portfolio cropped and resized

Hi there, I would like to use featured images for my portfolio on my website. I uploaded pictures with 3x4 ratio, landscape. However, they get resized to something in portrait and I cannot use the portfolio section of the theme like that, at all. Please check on: elisabethguggenbuehl.ch. Right on the top, it's called "Arbeiten" (works). The images should all be nicely 3x4. if you look at the horses for instance, you will instantly know what I mean: This is a picture of 2 horses beside each other and I would like to show it like that. How can I stop the cropping and resizing? It sometimes happens on other sections, too. Mainly if I change the featured image of post. But it's really annoying here. Thank you, and have a great weekend! Eilwen