In my wordpress post, I add the id "featured-image" (I know with later wordpress this is built-in, I'm on an old one).
<xmp><img src="http://mysite.com/catphoto.jpg" id="featured-image" /></xmp>
I write a meta tag for the og:image with a placeholder, such as my blog log. Add "id="meta-image" to the tag, i.e.
<meta property="og:image" src="http://mysite.com/logo.jpg" id="meta-image" />
var featured_image = $("#featured-image").attr("src");