Hi,
Your website is now having a jQuery Conflict that caused by K2 component, but i can't be sure if this is the reason why the last thumbnail is opened in the new window.
The infinite loop option of slider might conflict with something as well, i can't be sure. But try this;
Open this JS file:
www.igioiellidelmare.net/new/templates/ot_jewelry/html/com_virtuemart/assets/Flexible/additional-slider.js
and fin this line:
and replace it with this:
This will disable the infinite loop in the additional picture scroller. Check your website to see if this change fix the problem or not.
Regards.
Forum Support.