Facebook OG issue workaround

Hi – I am using this code

//Adding the Open Graph in the Language Attributes
function add_opengraph_doctype( $output ) {
		return $output . ' xmlns:og="http://opengraphprotocol.org/schema/" xmlns:fb="http://www.facebook.com/2008/fbml"';
	}
add_filter('language_attributes', 'add_opengraph_doctype');

//Lets add Open Graph Meta Info

function insert_fb_in_head() {
	global $post;
	if ( !is_singular()) //if it is not a post or a page
		return;
        echo '<meta property="fb:admins" content="YOUR USER ID"/>';
        echo '<meta property="og:title" content="' . get_the_title() . '"/>';
        echo '<meta property="og:type" content="article"/>';
        echo '<meta property="og:url" content="' . get_permalink() . '"/>';
        echo '<meta property="og:site_name" content="Your Site NAME Goes HERE"/>';
	if(!has_post_thumbnail( $post->ID )) { //the post does not have featured image, use a default image
		$default_image="http://example.com/image.jpg"; //replace this with a default image on your server or an image in your media library
		echo '<meta property="og:image" content="' . $default_image . '"/>';
	}
	else{
		$thumbnail_src = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'medium' );
		echo '<meta property="og:image" content="' . esc_attr( $thumbnail_src[0] ) . '"/>';
	}
	echo "
";
}
add_action( 'wp_head', 'insert_fb_in_head', 5 );

To get Facebook to recognize the content on my site. There is a problem with https though, and FB does not recognize the images (this has been discussed on many FB forums.

The workaround is to put the images into a subdomain of a separate domain – ok – so my default image shows now – good!

BUT – you can’t add a featured image through a URL – so to get the image for that post to show I want to create a custom field – and the value would be the image path in the sub-domain.

Here is my question – How would I modify the above code to pull the custom field data INSTEAD of the featured image data.

Thank you for all your help! You rock!!