functions.php

<?php

/**
 * Site Header Background 
 *
 * @author Bill Erickson
 * @link http://www.billerickson.net/code/background-image-site-header/
 */
function be_site_header_background( $attributes ) {
	$image = false;
	if( is_page() )
		$image = get_post_meta( get_the_ID(), 'be_page_header', true );
		
	if( ! $image )
		return $output;
		
	$image = wp_get_attachment_image_src( $image, 'full' );
	$attributes['class'] .= ' has-image';
	$attributes['style'] = 'background-image: url(' . $image[0] . ');';

	return $attributes;
}
add_filter( 'genesis_attr_site-header', 'be_site_header_background' );