I put a default entry for image in the open graph admin area. The URL is to my site's logo, so on any page that doesn't have an og:image generated, the default should show up (as per your docs).
On one of my pages, I put the code {SCOpenGraph image=} which appears to work... almost.
I now have TWO og:image tags on the page, one generated by the {SCOpenGraph} above, and one from the default. Removing the default entry on the back-end makes it go away.
The docs say that anything on the page will suppress the default, but it appears that it does not. Having two og:image tags on there causes Facebook to non-deterministically pick one, which is not good behavior.