Source

File: wp-admin/includes/class-custom-image-header.php

public function get_default_header_images() {
		$this->process_default_headers();

		// Get the default image if there is one.
		$default = get_theme_support( 'custom-header', 'default-image' );

		if ( ! $default ) { // If not, easy peasy.
			return $this->default_headers;
		}

		$default             = sprintf( $default, get_template_directory_uri(), get_stylesheet_directory_uri() );
		$already_has_default = false;

		foreach ( $this->default_headers as $k => $h ) {
			if ( $h['url'] === $default ) {
				$already_has_default = true;
				break;
			}
		}

		if ( $already_has_default ) {
			return $this->default_headers;
		}

		// If the one true image isn't included in the default set, prepend it.
		$header_images            = array();
		$header_images['default'] = array(
			'url'           => $default,
			'thumbnail_url' => $default,
			'description'   => 'Default',
		);

		// The rest of the set comes after.
		return array_merge( $header_images, $this->default_headers );
	}