Distorted pictures from link to thumbs of my JQuery-Slideshow

I’ve set up a JQuery-Slideshow on https://www.stardessert.ch/angebot/starpresent/

So far so good, it’s working on my PC and on my iPhone 4S :).
Unfortunately the link to the thumbnail-version is not so mobile friendly.

When opened up on my mobile I see the thumbnails but when clicking on a thumbnail for a bigger size the picture is completely distorted (on mobile only).
https://www.stardessert.ch/angebot/starpresent/nggallery/thumbnails

How can I define the size/display of these images (on mobiles)?

Thnx for tricks&hints:
Michelle