Source

File: wp-includes/customize/class-wp-customize-header-image-control.php

public function prepare_control() {
		global $custom_image_header;
		if ( empty( $custom_image_header ) ) {
			return;
		}

		add_action( 'customize_controls_print_footer_scripts', array( $this, 'print_header_image_template' ) );

		// Process default headers and uploaded headers.
		$custom_image_header->process_default_headers();
		$this->default_headers  = $custom_image_header->get_default_header_images();
		$this->uploaded_headers = $custom_image_header->get_uploaded_header_images();
	}