understrap/inc/custom-header.php

<?php
/**
 * Custom header setup.
 *
 * @package understrap
 */

add_action( 'after_setup_theme', 'understrap_custom_header_setup' );

if ( ! function_exists ( 'understrap_custom_header_setup' ) ) {
	function understrap_custom_header_setup() {

		/**
		 * Filter UnderStrap custom-header support arguments.
		 *
		 * @since UnderStrap 0.5.2
		 *
		 * @param array $args {
		 *     An array of custom-header support arguments.
		 *
		 *     @type string $default-image     		Default image of the header.
		 *     @type string $default_text_color     Default color of the header text.
		 *     @type int    $width                  Width in pixels of the custom header image. Default 954.
		 *     @type int    $height                 Height in pixels of the custom header image. Default 1300.
		 *     @type string $wp-head-callback       Callback function used to styles the header image and text
		 *                                          displayed on the blog.
		 *     @type string $flex-height     		Flex support for height of header.
		 * }
		 */
		add_theme_support( 'custom-header', apply_filters( 'understrap_custom_header_args', array(
			'default-image'      => get_parent_theme_file_uri( '/img/header.jpg' ),
			'width'              => 2000,
			'height'             => 1200,
			'flex-height'        => true,
		) ) );

		register_default_headers( array(
			'default-image' => array(
				'url'           => '%s/img/header.jpg',
				'thumbnail_url' => '%s/img/header.jpg',
				'description'   => __( 'Default Header Image', 'understrap' ),
			),
		) );
	}
}