global.js

jQuery(function($){

  // Preload header image
    var $header     = $( '.page-header' ),
        headerImage = $header.data( 'image' );

    if ( headerImage ) {
        var img = new Image();
        img.src = headerImage;

        $ (window ).load( function(){
            $header.css( 'background-image', 'url(' + headerImage + ')' );
        });
    }

});

page-header.php

<?php

/**
 * Page Header
 *
 */
function ea_page_header() {

	$image_id = $title = $content = false;
	if( is_singular() && has_post_thumbnail() )
		$image_id = get_post_thumbnail_id();

	if( is_page() ) {
		$title = get_the_title();
		remove_action( 'tha_entry_top', 'ea_entry_title' );
	}

	if( is_home() ) {
		$title = get_the_title( get_option( 'page_for_posts' ) );
	}

	if( is_archive() ) {
		$title = get_the_archive_title();
	}

	if( is_search() )
		$title = 'Search results for: <em>' . get_search_query() . '</em>';

	if( is_home() || is_archive() || is_search() )
		$image_id = get_post_thumbnail_id( get_option( 'page_for_posts' ) );

	if( empty( $image_id ) )
		$image_id = ea_cf( 'ea_header_image', false, array( 'type' => 'theme_option' ) );

	echo '<div class="page-header" data-image="' . wp_get_attachment_image_url( $image_id, 'full' ) . '"><div class="wrap">';
	if( !empty( $title ) )
		echo '<h1>' . $title . '</h1>';
	echo $content;
	echo '</div></div>';
}
add_action( 'tha_header_after', 'ea_page_header' );

style.css

.page-header {
	background-image: url(../images/loading.png);
	background-size: cover;
	background-position: center center;
	position: relative;
	transition: background-image 0.4s ease-in-out;
}