thumbnail url for FB open graph

FB social plugins when integrated to the site, sometimes pick up random thumbnails from among the existing images on the page

I am trying to integrate a social plugin to the product pages - now when I do that the plugin picks up a random image instead of the product thumbnail

To fix this problem, FB suggests that I insert the following tag in the section of the page

For this to appear on every product page I need to get the variable from the cart that holds the thumbnail URL so that I insert in the index.tpl

What is that variable?

Anyone any advice?