CoolRunning added a post in a topic: Sharing Things via Facebook - Images?
FaceBook uses the 'og:xxx' meta tags to determine image. If you analyse your page on FaceBook's developer tools page , you will see what FaceBook sees and uses.
For example, on our site it's seeing and using for the image:
Meta Tag <meta property="og:image" content=" http://www.coolrunni...meta_image.png" />
Which I'm just about to change for our own logo, in GlobalTemplate