Source

File: wp-includes/theme.php

function _get_random_header_data() {
	static $_wp_random_header = null;

	if ( empty( $_wp_random_header ) ) {
		global $_wp_default_headers;
		$header_image_mod = get_theme_mod( 'header_image', '' );
		$headers          = array();

		if ( 'random-uploaded-image' === $header_image_mod ) {
			$headers = get_uploaded_header_images();
		} elseif ( ! empty( $_wp_default_headers ) ) {
			if ( 'random-default-image' === $header_image_mod ) {
				$headers = $_wp_default_headers;
			} else {
				if ( current_theme_supports( 'custom-header', 'random-default' ) ) {
					$headers = $_wp_default_headers;
				}
			}
		}

		if ( empty( $headers ) ) {
			return new stdClass;
		}

		$_wp_random_header = (object) $headers[ array_rand( $headers ) ];

		$_wp_random_header->url           = sprintf( $_wp_random_header->url, get_template_directory_uri(), get_stylesheet_directory_uri() );
		$_wp_random_header->thumbnail_url = sprintf( $_wp_random_header->thumbnail_url, get_template_directory_uri(), get_stylesheet_directory_uri() );
	}

	return $_wp_random_header;
}