Tom,
Really not sure, in all honesty. Can you link (or Private Message us) a link to a page that has this issue?
Currently, the Open Graph image settings for K2 will only automatically pull the image that inserted into the article contents, it won't pull the image that's specifically set for the item in "Image" tab. That's something we're looking on, but not available even in 4.0.3. However, it sounds like when you use the WYSIWYG editor to add an image, the og:image tag is just the domain (no image). Is that correct?
Another thing to try is manually adding the image with the following tag in the article's contents:
{JFBCGraph image=http://site.com/link-to-image.jpg}
Hope that helps give some ideas, but again, if you can get me a URL to see the problem with, that may help me come up with other ideas.
Thanks,
Alex