backstretch-set.js

jQuery(function( $ ){

	$(".site-header").backstretch([BackStretchImg.src],{duration:3000,fade:750});

});

functions.php

<?php
//* Do NOT include the opening php tag

add_action( 'wp_enqueue_scripts', 'sk_enqueue_backstretch' );
/**
 * Set Featured Image as Header's background using Backstretch on singular entries.
 * For singular entries not having a Featured Image and other sections of the site,
 * like category archives, a default image will be set as Header's background.
 *
 * @author Sridhar Katakam
 * @link   http://sridharkatakam.com/set-featured-image-headers-background-using-backstretch-genesis/
 */
function sk_enqueue_backstretch() {

	wp_enqueue_script( 'backstretch', get_stylesheet_directory_uri() . '/js/jquery.backstretch.min.js', array( 'jquery' ), '', true );
	wp_enqueue_script( 'backstretch-set', get_stylesheet_directory_uri() . '/js/backstretch-set.js' , array( 'backstretch' ), '1.0.0', true );

	if ( has_post_thumbnail() && is_singular() ) {
		$featured_image_url = wp_get_attachment_url( get_post_thumbnail_id() );
		$backstretch_src = array( 'src' => $featured_image_url );
	} else {
		$default_header_url = get_stylesheet_directory_uri() . '/images/header-default.jpg';
		$backstretch_src = array( 'src' => $default_header_url );
	}

	wp_localize_script( 'backstretch-set', 'BackStretchImg', $backstretch_src );

}