Change image when sharing on facebook

When I share the link (http://peaceconference.no/) of my site on facebook I only get a standard image that loads. How can I change this image?

I read this article but did not understand the solution: http://www.gavick.com/magazine/image-fr ... aring.html

Facebook get this data automatically and you don't have so many options to change it, you can try to use Open Graph and install dedicated plugin from rest_files package. More informations about Open Graph you will find here » https://developers.facebook.com/docs/web/share