WP_Customize_Header_Image_Setting::update( $value )

	public function update( $value ) {
		global $custom_image_header;

		// If _custom_header_background_just_in_time() fails to initialize $custom_image_header when not is_admin().
		if ( empty( $custom_image_header ) ) {
			require_once( ABSPATH . 'wp-admin/custom-header.php' );
			$args = get_theme_support( 'custom-header' );
			$admin_head_callback = isset( $args[0]['admin-head-callback'] ) ? $args[0]['admin-head-callback'] : null;
			$admin_preview_callback = isset( $args[0]['admin-preview-callback'] ) ? $args[0]['admin-preview-callback'] : null;
			$custom_image_header = new Custom_Image_Header( $admin_head_callback, $admin_preview_callback );
		}

		// If the value doesn't exist (removed or random),
		// use the header_image value.
		if ( ! $value )
			$value = $this->manager->get_setting('header_image')->post_value();

		if ( is_array( $value ) && isset( $value['choice'] ) )
			$custom_image_header->set_header_image( $value['choice'] );
		else
			$custom_image_header->set_header_image( $value );
	}