Add a parallax image behind a page entry header in Genesis - pure css solution. (You'll need to style your image to be tall enough to match the webpage header, so it is behind the main page header on scroll)

functions.php

<?php
/**
 * Add a parallax image behind a page entry header in genesis.
 * 
 * @author Joshua David Nelson, josh@joshuadnelson.com
 */

// Parallax Image
add_action( 'genesis_header', 'parallax_image', 15 );
/**
 * Parallax Image
 *
 * @since 1.0.0
 */
function parallax_image() {
	global $post;
	if( isset( $post->ID ) && is_page()  ) {
		$thumb = has_post_thumbnail( $post->ID );
		if( $thumb ) {
			$thumbnail_src = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), array( 1500, 300 ) );
			if( $thumbnail_src )
				echo '<div class="parallax_image" style="background-image: url(' . $thumbnail_src . ');"></div>';
		}
	}
}

// Featured image behind header
add_action( 'genesis_meta', 'jdn_featured_image_header' );
/**
 * Display the featured image behind the header.
 *
 * @since 1.0.0
 */
function jdn_featured_image_header() {
	if( is_page() ) {
		$thumb = has_post_thumbnail( get_the_ID() );
		if( $thumb ) {
			remove_action( 'genesis_entry_header', 'genesis_entry_header_markup_open', 5 );
			remove_action( 'genesis_entry_header', 'genesis_do_post_title' );
			remove_action( 'genesis_entry_header', 'genesis_entry_header_markup_close', 15 );
			add_action( 'genesis_after_header', 'genesis_entry_header_markup_open', 15 );
			add_action( 'genesis_after_header', 'genesis_do_post_title', 20 );
			add_action( 'genesis_after_header', 'genesis_entry_header_markup_close', 25 );
		}
	}
}

style.scss

/* Styles needed for parallax effect */
.site-header {
	.wrap {
		position: relative;
	}
}
.parallax_image {
	position: fixed;
	top: 0;
	left: 0;
	background-repeat: no-repeat;
	min-width: 100%;
	width: 100%;
	@include background-size( cover );
	background-position: center top;
	height: 500px;
	z-index: -1;
}