Pinboard featured image blurry

Hi! I’m using the pinboard theme but I have a problem with the featured images that you can set up for each blog entry.
It seems that you always have to use square images or else they will become blurry. Is this right or am I just doing something else wrong?
Is there any way to use other images as featured without them being blurry?

Here is my page:
http://anshie.de/

The image (the collage) of the first entry looks fine, but if you scroll down you can easly see my problem.

Thank you in advance!