Drupal 6 theme functions to insert a jcarousel that uses lightbox into any node with the appropriate image field, for use in a template.php file

template.php

<?php
/**
 * Node preprocess function
 */
function mytheme_preprocess_node(&$vars, $hook) {
  if ($vars['node']->field_image && !empty($vars['node']->field_image)) {
    $vars['image_carousel'] = _mytheme_generate_carousel($vars['node']->field_image);
  }
}

/**
 * Helper function to get the appropriate imagecache variant of an image
 *
 * This makes sure that the imagecached version of the image exists, and if it does
 * not creates it.
 *
 * @param $filepath
 *   The path to the original file
 * @param $presetname
 *   The imagecache preset to use
 * @return
 *   The path to the cached version of the image
 */
function _mytheme_generate_imagecache(&$filepath, $presetname = 'teaser') {

  // First, check that the preset actually exists
  $preset = imagecache_preset_by_name($presetname);
  if (!$preset) {
    return FALSE;
  }

  // Try to get the existing cached image
  $dst = imagecache_create_path($presetname, $filepath);
  $cache_path = $dst;

  if (!file_exists($dst) || is_dir($dst)) {
    imagecache_build_derivative($preset['actions'], $filepath, $cache_path);
  }

  return $cache_path;
}

/**
 * Helper function to generate a jcarousel from a cck image field
 *
 * @param $pics
 *   The cck image field
 * @return
 *   The images contained in the array, in a carousel or item list
 */
function _mytheme_generate_carousel ($pics) {

  $carousel = array();
  $lnk_options = array(
    'html' => TRUE,
    'attributes' => array(
      'class' => 'carousel-item',
    ),
  );

  // Use Lightbox if available
  if (function_exists('lightbox2_menu')) {
    $lnk_options['attributes']['rel'] = 'lightbox';
  }

  foreach ($pics as $item) {
    $thumb = _mytheme_generate_imagecache($item['filepath'], 'carousel');
    $image = _mytheme_generate_imagecache($item['filepath'], 'large');

    $carousel[] = l("<img src='/{$thumb}' alt='' title='{$item['filename']}'/>", $image, $lnk_options);
  }

  // Theme the feed as a carousel if the module jcarousel is available
  if (function_exists('theme_jcarousel')) {
    $options = array (
      'buttonNextEvent' => 'click',
      'buttonPrevEvent' => 'click',
      'visible' => 4,
      'scroll' => 3,
    );
    return theme('jcarousel', $carousel, $options);
  }
  else {
    return theme('item_list', $carousel);
  }
}

Thank you for that snippet! Got me 90% there :)