I actually already got it working. I changed the size in functions.php.
I circumvented it by using batchediting in Photoshop. I made all the photos square by changing the the size of the canvas.
This works excellent for the thumbnail as there is no cutting of going on. Strangely enough if click on more info, the picture in that view is not square apparantly because it does some clipping on top and bottom. Then if you click on that picture you get a pop-up with a perfectly square photo again.
Any idea what’s going on?