For a bit of follow-up, the problem is that WordPress SEO automatically adds an og:image for you, which is great. But in what is probably its most useful case, Facebook, they require an image with a minimum height of 200px. Read here. http://developers.facebook.com/docs/concepts/opengraph/distribution/#image
A bit of control over what the plugin uses for the og image would be nice. At the moment if you pass Facebook an undersized image with og:image the scraper just gives up and you lose the benefit of the remaining valid og tags.