[Plugin: Multiple Post Thumbnails] Use secondary image for FB Open Graph

I’m using Facebook’s new WP plugin. It used the featured image in the Open Graph metadata. I’d like to change this to the secondary image. I’m a PHP novice and am not sure what to change in the following code. Thanks.

if ( post_type_supports( $post_type, ‘thumbnail’ ) && function_exists( ‘has_post_thumbnail’ ) && has_post_thumbnail() ) {
list( $post_thumbnail_url, $post_thumbnail_width, $post_thumbnail_height ) = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), ‘full’ );

if ( ! empty( $post_thumbnail_url ) ) {
$image = array( ‘url’ => $post_thumbnail_url );

if ( ! empty( $post_thumbnail_width ) )
$image[‘width’] = absint( $post_thumbnail_width );

if ( ! empty($post_thumbnail_height) )
$image[‘height’] = absint( $post_thumbnail_height );

$meta_tags[‘http://ogp.me/ns#image’%5D = array( $image );
}
}
}

http://ww.wp.xz.cn/extend/plugins/multiple-post-thumbnails/