Super-simple fall-back open graph image for your WordPress blog. I didn't want/need a whole plugin for this, just wanted a simple og:image fall-back for my site.

basic-og-image-fallback.php

<?php
// Add this to your functions.php file
 
function mgr_og_image_fallback() {
	if (!has_post_thumbnail()) {
		// Replace the image URL unless you want my mug as your image for Open Graph Image ;)
		?>
		<meta property="og:image" content="http://i1.wp.com/matthewgrichmond.com/files/2012/03/matt.jpg?resize=310%2C180" />
		<?php
	}
}
add_action('wp_head', 'mgr_og_image_fallback');