WordPress: Image Custom Meta Property

Asset.php

add_action( 'add_meta_boxes', 'custom_meta_boxes' );
add_action( 'save_post', 'ExtraImageProperty_save' );

function custom_meta_boxes() {
	add_meta_box('ExtraImageProperty', 'Extra Image Property', 'ExtraImageProperty', 'page');
}

function ExtraImageProperty() {
	global $post;
  $extra_image_url = get_post_meta($post->ID, 'extra_image_url', true);
  wp_nonce_field( plugin_basename( __FILE__ ), 'ExtraImageProperty_wpnonce_name' );
	?>
	<script type="text/javascript" charset="utf-8">
		jQuery(document).ready(function() {
		var image;
			
		 jQuery('#upload_image_button').click(function() {
		  tb_show('', 'media-upload.php?type=image&amp;TB_iframe=true');
		  return false;
		 });

		 jQuery('#remove_image_button').click(function() {
		 	jQuery('#extra_image_url').val("");
  		jQuery('#image_display').toggle();
			jQuery('#upload_image_button').toggle();
			jQuery('#remove_image_button').toggle();
		 });

		 window.send_to_editor = function(html) {
		  img = jQuery('img',html);
			jQuery('#extra_image_url').val(img.attr('src'));
			jQuery('#image_display').attr("src",img.attr('src'));
			jQuery('#image_display').toggle();
			jQuery('#upload_image_button').toggle();
			jQuery('#remove_image_button').toggle();
		  tb_remove();
		 }
		});
	</script>
	<?php
?>
	<input id="extra_image_url" name="extra_image_url" type="hidden" value="<?php echo $extra_image_url?>" />
	<img <?php if (empty($extra_image_url)) echo 'style="display:none"' ?> id="image_display" src="<?php echo $extra_image_url?>" width="266" alt="" /> 
	<p><a <?php if (!empty($extra_image_url)) echo 'style="display:none"' ?> title="Set extra image" id="upload_image_button" href="#" />Set extra image</a></p>
	<p><a <?php if (empty($extra_image_url)) echo 'style="display:none"' ?> title="Remove extra image" id="remove_image_button" href="#" />Remove extra image</a></p>
	<?php
}

function ExtraImageProperty_save( $post_id ) {
  if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) 
      return;
  if ( !wp_verify_nonce( $_POST['ExtraImageProperty_wpnonce_name'], plugin_basename( __FILE__ ) ) )
      return;
  if ( 'page' == $_POST['post_type'] ) {
    if ( !current_user_can( 'edit_page', $post_id ) )
        return;
  } else {
    if ( !current_user_can( 'edit_post', $post_id ) )
        return;
  }
	update_custom_meta($post_id, $_POST['extra_image_url'], 'extra_image_url');
}

function update_custom_meta($postID, $newvalue, $field_name) {
	if (!get_post_meta($postID, $field_name)) {
		add_post_meta($postID, $field_name, $newvalue);
	} else {
		update_post_meta($postID, $field_name, $newvalue);
	}
}