Simple catch – Facebook thumbnail

Hi there, When sharing my posts on Facebook, I am unable to get a thumbnail image to appear, with the exception of one post, which displays my website's banner image. I've installed Wordpress SEO and tried setting a specific image to be displayed as a thumbnail on Facebook. Still nothing. Would really appreciate some help. Blog osts in question can be seen at: http://www.designdevelopinspire.ca/blog/ Thanks, Ian