When you share a post on Facebook, or when Jetpack Publicize publishes a post to your Facebook page, Facebook crawls the page and looks for Open Graph meta tags in the head to build a complete post preview (with an image, title, description, ...).
You can see what Facebook sees by entering one of your posts' URLs in Facebook Debugger:
On your site, you have actually added 2 sets of Facebook Open Graph meta tags;
- one is added by the WordPress SEO plugin, and contains correct information (with images)
- The other set is added by another plugin, Shareaholic, and only contains an image tag, that is already added by WordPress SEO. I would consequently suggest that you remove it.
It's also worth noting that Facebook only grabs images that are larger than 200px x 200px. If the images mentioned in your Open graph meta tags are smaller, they won't appear on Facebook.
I believe that's what happened with your example post: it includes the following image meta tag:
As you can see, this image is 1px x 1px, and is consequently ignored by Facebook.
I hope this helps. If you have more questions about how WordPress SEO or Shareaholic grab images and define the open graph meta tags, I would suggest that you contact the plugin authors about this.