Include thumbnail in WP RSS

functions.php

<?php
function dorphin_rss_item()
{
	global $post;
	$d = get_post_custom();

	if (!empty($d['thumb_image']))
	{
		?>
			<enclosure url="<?php echo(CDN_IMAGE.$d['thumb_image'][0]); ?>" type="image/jpeg" />
			<media:content url="<?php echo(CDN_IMAGE.$d['thumb_image'][0]); ?>" type="image/jpeg" expression="sample" />
			<media:thumbnail url="<?php echo(CDN_IMAGE.$d['thumb_image'][0]); ?>" type="image/jpeg" width="280" height="158" />
		<?php
	}
}
add_action('rss2_item', 'dorphin_rss_item');
?>