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<?php
if (function_exists('add_theme_support'))
{
    // Add Thumbnail Theme Support
    add_theme_support('post-thumbnails');
    
    // Add sizes
    add_image_size('lightbox', 2000, '', true); 

}

// Add sizes to backend
function my_custom_sizes( $sizes ) {
  return array_merge( $sizes, array(
	'lightbox' => __( 'Lightbox' ),
  ) );
}
add_filter( 'image_size_names_choose', 'my_custom_sizes' );

?>

thumbnail       // Thumbnail (default 150px x 150px max)
medium          // Medium resolution (default 300px x 300px max)
large           // Large resolution (default 640px x 640px max)
full            // Original image resolution (unmodified)