gistfile1.php

<?php

// Adding option to use featured image (if it exists)
// in place of background image

remove_action( 'genesis_after_header', 'beautiful_site_header_banner'); // removes existing banner image

add_image_size( 'featured-banner', 2000, 200, TRUE ); // creates a featured image size for the banner

function amcs_featured_img() {
	if ( has_post_thumbnail() ) { // checks post has thumbnail
	// gets URL for that image
	$background = wp_get_attachment_image_src( get_post_thumbnail_id( $page->ID ), 'featured-banner' ); 
	if (is_array($background)) {   
    // thanks to @magicroundabout for the fix
    // echo the output
	echo '<div class="featured-header-banner"  style="background: url(' ;
	echo $background[0];
	echo ') no-repeat scroll center center #FFFFFF"></div>'; 
	}}
	else { // if no featured image, adds class to use existing banner
	echo '<div class="site-header-banner"></div>';
}}

add_action( 'genesis_after_header', 'amcs_featured_img' );