Is this happening on single blog posts for you? I had a problem getting any page other than the homepage to show an image property, so I looked at the Opengraph class and it looks like if a single post has no attachments etc it will not add an image. I added the default image code from class-opengraph.php right after the if(is_singluar()) test for the image function and it works for me now.
After seeing your website, it may still be relevant as I think pages are technically singular. Even though if you have a page set as the homepage it should have added the frontpage image.
