thumbnail external link

Hi,

I’ve got this code that shows the post thumbnail.
In the back-end the post has link format.
I would like that in the front-end, the thumbnail has the link I put in the back-end (an external link), and not go to the post link.

I don’t know how to change the a href.

function post_thumbnail() {
		global $post;
		if (has_post_thumbnail($post->ID)) {
			echo '<a href="'.get_permalink($post->ID).'" target="blank">'.$this->get_thumbnail( $layout ).'</a>

Thanks a lot.
D.