Get the post thumbnail url of a post in Wordpress

functions-thumbnail.php

<?php 

/**
 * Return the post thumbnail url of the content (use in loop)
 * by Andrea Rufo www.andrearufo.it
 * 
 * @param  string $size the image size registered in theme via add_image_size() function
 * @param  int $id the ID of the post
 * @return string url of the image
 */
function ar_post_thumbnail_url($size = 'large', $id = null){

	// set the ID of the post
	if( is_null($id) )
		$id = $post->id;

	// check if the post has a featured image
	if( !has_post_thumbnail() )
		return false;

	// get the featured image id
	$thumb_id = get_post_thumbnail_id( $id );

	// get the info of attachment
	$image = wp_get_attachment_image_src( $thumb_id, $size );

	// return the url
	return $image[0];
}

loop.php

<?php if ( has_post_thumbnail() ): ?>

  <div class="featured-image" style="background-image: url(<?php echo ar_post_thumbnail_url('large') ?>)"></div>

<?php endif; ?>