Change thumbnail sizes in the Broadsheet theme

broadsheet-thumnail-size.php

<?php
/**
 * Plugin Name: Change Archive Image Thumbnail size.
 * Plugin URI: https://prothemedesign.com
 * Description: Change the size of the archive image thumbnails so that they can be made full width.
 * Author: Ben Gillbanks
 * Version: 1.0
 * Author URI: https://prothemedesign.com
 * Text Domain: broadsheet
 */

function broadsheet_archive_image_size() {

  // Post listing image size.
	add_image_size( 'broadsheet-archive', 200, 160, true );

  // Featured image size.
	add_image_size( 'broadsheet-feature', 260, 163, true );

}

add_action( 'after_setup_theme', 'broadsheet_archive_image_size', 99 );