Example custom code plugin to change header sizes set by theme

readme.md

Install customcode.php in your plugin directory or better create a subdirectory called custom code and put this in there. And activate the plugin.

Change names and sizes as required

After installing you will need to regenerate thumbnails - there is a plugin that does that https://en-gb.wordpress.org/plugins/regenerate-thumbnails/

Use the structure to add other custom tweaks without changing teh them's functions.php

Not tested - but should work - any problems let me know so I can tweak...

customcode.php

<?php
/**
 *
 * Plugin Name:       Custom Code
 * Plugin URI:        https://fullworks.net/products/custom-plugin-development/
 * Description:       Custom Code
 * Version:           0.0.0
 * Author:            Fullworks
 * Author URI:        http://fullworks.net/
 * License:           GPL-3.0+
 * License URI:       http://www.gnu.org/licenses/gpl-3.0.txt
 *
 */


// If this file is called directly, abort.
if ( ! defined( 'WPINC' ) ) {
	die;
}

add_action( 'after_setup_theme', function () {
	remove_image_size( 'header-image' );    // change this to remove the already theme registered size - change the name as required
	add_image_size( 'header-image', 1600, 600, true ); // cahnge the width, height and crop as required
}, 99 );