WordPress page header image on custom post type archive template.

header.php

<?php

if ( is_post_type_archive() ) {
	$object = get_queried_object();

	if ( isset( $object->rewrite ) && is_array( $object->rewrite ) && isset( $object->rewrite['slug'] ) ) {
		$slug = $object->rewrite['slug'];

		$page = get_page_by_path( $slug );

		if ( null !== $page ) {
			echo get_the_post_thumbnail( $page->ID, 'header' );
		}
	}
}