Function that returns the URL of a post thumbnail in Wordpress inside the loop. Used for displaying FlexSlider 2 thumbnail navigation.

Return Thumbnail URL

function prefix_thumbnail_url( $postid ) {
          		
	$post_image_id = get_post_thumbnail_id( $postid );
	$thumbnail = wp_get_attachment_image_src( $post_image_id, $image-size, false);	        
	if ($thumbnail) (string)$thumbnail = $thumbnail[0];
	return $thumbnail;
	
}