Google+ button is not using the specified opengraph image. Facebook does.
I've seen one post about the image size being a factor.
Is there any documentation that specifies the correct image size to use with google+ and opengraph?
Google+ FAQ: Why isn't my +Snippet image appearing in the share bubble?
Images that are too small or not square enough will not be included in the +Snippet, even if explicitly referenced using schema.org microdata or Open Graph markup. Specifically, the height must be at least 120px, and if the width is less than 100px, then the aspect ratio must be no greater than 3.0.