Default Header Images Generator

<?php
/**
 * Default header images for the theme.
 *
 * These appear as choices under Appearance → Header. They are theme files, not uploads: the
 * paths are relative to the theme directory, and register_default_headers() does nothing at all
 * unless the theme also declares custom-header support.
 */

/**
 * Declares header support and registers the choices.
 */
function my_theme_custom_header_setup() {
	add_theme_support(
		'custom-header',
		array(
			'default-image'          => get_template_directory_uri() . '/assets/headers/mountains.jpg',
			'width'                  => 1600,
			'height'                 => 480,
			// Without this the admin offers a cropper that will not let the user keep their own
			// aspect ratio, which is the right trade only if the design truly requires the size.
			'flex-width'             => true,
			'flex-height'            => true,
			'uploads'                => true,
			'header-text'            => true,
			'default-text-color'     => 'ffffff',
			'wp-head-callback'       => 'my_theme_header_style',
		)
	);

	// The %s in these paths is filled in by WordPress with the theme directory URI. Using a
	// hardcoded URL here breaks the moment the site moves domain or the theme is renamed.
	register_default_headers(
		array(
			'mountains' => array(
				'url'           => '%s/assets/headers/mountains.jpg',
				'thumbnail_url' => '%s/assets/headers/mountains-thumb.jpg',
				'description'   => _x( 'Mountains at dawn', 'header image description', 'my-theme' ),
			),
			'harbour' => array(
				'url'           => '%s/assets/headers/harbour.jpg',
				'thumbnail_url' => '%s/assets/headers/harbour-thumb.jpg',
				'description'   => _x( 'Harbour in winter', 'header image description', 'my-theme' ),
			),
		)
	);
}
add_action( 'after_setup_theme', 'my_theme_custom_header_setup' );

/**
 * Prints the header styles.
 *
 * Registered as wp-head-callback above. Without a callback the header image is registered and
 * never rendered, which is the commonest reason "the image does not show up".
 */
function my_theme_header_style() {
	$image = get_header_image();

	if ( ! $image ) {
		return;
	}

	?>
	<style id="my_theme-header-css">
		.site-header {
			background-image: url( <?php echo esc_url( $image ); ?> );
			background-position: center;
			background-size: cover;
			min-height: 480px;
		}
	</style>
	<?php
}