Add image sizes on the front end only

add_image_size.php

add_action( 'after_setup_theme', 'front_end_image_sizes' );
function front_end_image_sizes() {
  //Only register these on the front end for our templates 
  if ( !is_admin() && ( !defined( 'DOING_AJAX' ) || !DOING_AJAX ) ) :
    add_image_size( 'hero', 2000, 1000, false );
    add_image_size( 'smallish', 400, 400, false );
    add_image_size( 'tweener', 900, 700, false );
  endif;
}