How do I make the product page pull out a full size image, rather than a popup?

In your templates folder, under "templates", there is a file called tpl_modules_main_product_image.php.

Change this bit...

<script language="javascript" type="text/javascript"><!--
document.write('<?php echo '<a href="javascript:popupWindow(\\\'' . zen_href_link(FILENAME_POPUP_IMAGE, 'pID=' . $_GET['products_id']) . '\\\')">' . zen_image($products_image_medium, addslashes($products_name), MEDIUM_IMAGE_WIDTH, MEDIUM_IMAGE_HEIGHT) . '<br /><span class="imgLink">' . TEXT_CLICK_TO_ENLARGE . '</span></a>'; ?>');
//--></script>

to...

<?php echo zen_image($products_image_medium, addslashes($products_name), MEDIUM_IMAGE_WIDTH, MEDIUM_IMAGE_HEIGHT); ?>