Source

File: wp-includes/theme.php

function get_custom_header() {
	global $_wp_default_headers;

	if ( is_random_header_image() ) {
		$data = _get_random_header_data();
	} else {
		$data = get_theme_mod( 'header_image_data' );
		if ( ! $data && current_theme_supports( 'custom-header', 'default-image' ) ) {
			$directory_args        = array( get_template_directory_uri(), get_stylesheet_directory_uri() );
			$data                  = array();
			$data['url']           = vsprintf( get_theme_support( 'custom-header', 'default-image' ), $directory_args );
			$data['thumbnail_url'] = $data['url'];
			if ( ! empty( $_wp_default_headers ) ) {
				foreach ( (array) $_wp_default_headers as $default_header ) {
					$url = vsprintf( $default_header['url'], $directory_args );
					if ( $data['url'] == $url ) {
						$data                  = $default_header;
						$data['url']           = $url;
						$data['thumbnail_url'] = vsprintf( $data['thumbnail_url'], $directory_args );
						break;
					}
				}
			}
		}
	}

	$default = array(
		'url'           => '',
		'thumbnail_url' => '',
		'width'         => get_theme_support( 'custom-header', 'width' ),
		'height'        => get_theme_support( 'custom-header', 'height' ),
		'video'         => get_theme_support( 'custom-header', 'video' ),
	);
	return (object) wp_parse_args( $data, $default );
}