Apparently, Facebook doesn’t like .png images. When I tried to defined a png in the open graph properties “og:image”, the loading icon in the share window goes on for awhile before disappearing entirely. Fix: use a .jpg instead.
As advised on facebook docs: og:image – An image URL which should represent your object within the graph. The image must be at least 50px by 50px and have a maximum aspect ratio of 3:1.
Also, consider using the handy linter utility at http://developers.facebook.com/tools/lint/ to check the meta data of your page.