No help for thick box, but just to let you know a little more about slimbox2 and what John said about it being simpler and quicker(I think).
I had been struggling to install slimbox2. but eventually I used the install for lightbox
http://addons.oscommerce.com/info/6517
to get the code change in the body section of the products_info.php which is the same for slimbox2.
Basically it was
Find this code in products_info.php
document.write('<?php echo '<a href="java script:popupWindow(\'' . tep_href_link(FILENAME_POPUP_IMAGE, 'pID=' . $product_info['products_id']) . '\')">' . tep_image(DIR_WS_IMAGES . $product_info['products_image'], addslashes($product_info['products_name']), SMALL_IMAGE_WIDTH, SMALL_IMAGE_HEIGHT, 'hspace="5" vspace="5"') . '<br>' . TEXT_CLICK_TO_ENLARGE . '</a>'; ?>');
Replace it with
document.write('<?php echo '<a href="images/' . $product_info['products_image'] . '" rel="lightbox" title="' . $product_info['products_name'] . '">' . tep_image(DIR_WS_IMAGES . $product_info['products_image'], addslashes($product_info['products_name']), SMALL_IMAGE_WIDTH, SMALL_IMAGE_HEIGHT, 'hspace="5" vspace="5"') . '<br>' . TEXT_CLICK_TO_ENLARGE . '</a>'; ?>');
The three lines in the HEAD section for slimbox2 are (for me)
<script type="text/javascript" src="js/jquery.js"></script>
<script type="text/javascript" src="js/slimbox2.js"></script>
<link rel="stylesheet" href="css/slimbox2.css" type="text/css" media="screen" />
Instructions on the files and locations required in the js and css folder are here
http://www.digitalia.be/software/slimbox2
Only other issue I had was that when downloading and installing the jquery file from jquery.com, which is needed for slimbox2, the filename needed changing to jquery.js rather than the name that downloaded (jquery-1.3.2.min.js )
I am only using it with single images.
Andy