Ok, so I found a bug which I believe is with WP SEO by Yoast and not with the other plugin.
I'm using a lazy loading plugin for WordPress - http://wordpress.org/extend/plugins/lazy-load/
I was trying to get rid of Simple Facebook Connect, and let WP SEO by Yoast handle my social stuff, but it doesn't grab any images in the og:image tags.
When I use Simple Facebook Connect, it is able to grab the og:image tags just fine. Could it be that the lazy-load plugin replaces the img src=with a 1px image and use data-src for the actual image. Otto's SFC plugin can detect it, but WP SEO by Yoast cannot.