WP: add custom image sizes

wp_add_custom_image_sizes.php

<?php

/**
 * Add custom image sizes.
 *
 * @hook after_setup_theme
 *
 * @return void
 */
function my_theme_add_custom_image_sizes() : void
{
  add_image_size( 'my-theme-thumbnail', 600, 450, true );
  add_image_size( 'my-theme-medium', 900, 675, true );
  add_image_size( 'my-theme-large', 1200, 900, true );
}
add_action( 'after_setup_theme', 'my_theme_add_custom_image_sizes' );

/**
 * Make custom image sizes selectable
 *
 * @hook image_size_names_choose
 *
 * @param  array $sizes
 * @return array
 */
function my_theme_select_custom_image_sizes( array $sizes ) : array
{
  return array_merge( $sizes, [
    'my-theme-thumbnail' => __( 'Card Image' ),
    'my-theme-medium'    => __( 'Section Image' ),
    'my-theme-large'     => __( 'Background Image' )
  ]);
}
add_filter( 'image_size_names_choose', 'my_theme_select_custom_image_sizes' );