define & use custom image sizes

image-sizes.php

<?php
// $src = wp_get_attachment_image_src( $attachment_id, 'store-archive' );
// $src[0] = "http://.../uploads/my_image-240x350.jpg"
// $src[1] = 240
// $src[2] = 350

add_action( 'after_setup_theme', 'ld_theme_image_sizes' );

function ld_theme_image_sizes() {
  // archive pages
  add_image_size( 'store-archive', 240, 350, 1 );
  
  // single product pages, the main image
  add_image_size( 'store-singular', 520, 700, 1 );
  
  // single product pages, the thumbnails used below the main image
  add_image_size( 'store-gallery-thumb', 100, 150, 1 );
}