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, ...). Jetpack automatically adds these open graph meta tags for you.
I checked your latest post, and the tags are properly added:
http://i.wpne.ws/Q0L8
You can see what Facebook sees by entering one of your posts' URLs in Facebook Debugger:
https://developers.facebook.com/tools/debug
https://developers.facebook.com/tools/debug/og/object?q=http%3A%2F%2Fwww.minecraftbuildinginc.com%2Fcity-of-maikura-japanese-metropolis1069%2F
http://i.wpne.ws/PztP
For some reason, Facebook returns an error:
Provided og:image is not big enough. Please use an image that's at least 200x200 px. Image 'http://www.minecraftbuildinginc.com/wp-content/uploads/2013/07/City-of-Maikura-Minecraft-building-city-towers.jpg' will be used instead.
it's a bit weird since the alternative image Facebook wants to use is the one that is specified as an Open Graph meta tag. And it is larger than 200x200 px.
That seems to be a bug with Facebook. I would suggest that you report it to the Facebook team here:
https://developers.facebook.com/bugs