That's the issue with Facebook... there really is no "formula" per sey of what images that Facebook will scrape and provide you to select as a thumbnail for the page.
There are ways' to "skew" what they produce, but you really have to play with the images and placement on your pages - play with the sizes of those images, etc. I had a page that had (litterally) over 90 images on it... it was PLAIN HTML for the page, and all the images were just a plethora of photos I took at my daughters play.
I posted the link on Facebook, and it gave me the option for three images to use as thumbnails (none of which had anythign to do with the photos on the page I had created - instead, were the three 20px x 20px buttons to go from one page to the next... not even my logo which was 234x60 at the top of the page was made available to use for the thumbnail.
Now, considering my past experience with forum support here, you are probably not going to get very far with troubleshooting this problem as it really isn't a 'bug' in the template, and its more of you wanting to better manipulate what a 3rd party is scraping from your pages. (just sayin - you might not want to get your hopes up)
Good luck with the effort though... my advice... play with the primary images on whatever page you are trying to get them to scrape...