Ah, the problem is that you published that post as an Image post type. Posts published on WordPress.com with an Image post type are sent to Facebook as an Image item, so they effectively scramble to figure out which image to feature and wind up with that mess (WordPress.com Image post types and Facebook Image items are designed for one image only).
I noticed that you have the Image post type set as your default at Settings -> Writing in your blog's Dashboard. You might want to consider changing that, or at least manually selecting the Standard post type before publishing anything that is not just a single image.