Facebook has sporadic issues with picking up the image when you try to post a link. I usually post sometimes as many as five a day to my Facebook fan page (facebook.com/TheArtOfDanielEskridge), and I usually encounter this on average every two days or so. Sometimes, all you can do is wait for Facebook to get their act together and try again after a few hours.
However, if you are not that patient, like me, you can try going to http://goo.gl shortening the link, then posting that Facebook instead. Usually this get the picture to show for me when FB is acting up.