WordPress at a glance
function is not described

header_image() WP 2.1.0

Display header image URL.

No Hooks.

Return

Nothing (null).

Usage

header_image();

Code of header image: wp-includes/theme.php VER 5.1.1

<?php
function header_image() {
	$image = get_header_image();
	if ( $image ) {
		echo esc_url( $image );
	}
}