function add_fb_meta() {
  global $post;
  if (get_post_meta($post->ID, 'image_src', true)) { ?>
<link rel="image_src" href="<?php echo get_post_meta($post->ID, 'image_src', true); ?>" / >
<?php
  }
}
add_action('wp_head', 'add_fb_meta');