Facebook Sharing - What content gets chosen?

I've just been running some tests about how the Facebook share icon works in terms of what image, title, and message it picks up. I'm pretty sure that when you put a share icon on a webpage, as we've done on so many of the T4L webpages, here's one for example:
http://www.time4learning.com/secular/index.shtml
the share icon picks up the page title, the meta description and the first image on the page. Here's a test: 

 
However, I've been trying to figure out how Facebook picks the image. In this case, it gives the user a choice of 12, all of which can be found on the page. The first one is an odd choice, it's not the first image on the page. It might be the largest. It's not the one that's closest to the top in the center either.