Añadir tamaños de imagenes (miniaturas) a WordPress y dar la opción de elegirlas al añadirlas a un post

add-image-sizes.php

<?php
function jpd_add_image_sizes() {
    add_image_size( 'pw-thumb', 300, 100, true );
    add_image_size( 'pw-large', 600, 300, true );
}
add_action( 'init', 'jpd_add_image_sizes' );
 
function jpd_show_image_sizes($sizes) {
    $sizes['pw-thumb'] = __( 'Custom Thumb', 'perezdans' );
    $sizes['pw-large'] = __( 'Custom Large', 'perezdans' );
 
    return $sizes;
}
add_filter('image_size_names_choose', 'jpd_show_image_sizes');