You would have to clean this up but if you look in the tpl_index_product_list.php you can see where I insert a quick peice of code to grab the image and display it:
<?php
if (PRODUCT_LIST_CATEGORIES_IMAGE_STATUS == 'true') {
// parent image
$check_categories_parent_id = $db->Execute("SELECT parent_id FROM " . TABLE_CATEGORIES . " WHERE categories_id='" . $current_category_id . "'");
$catgories_parent_image = zen_get_categories_image($check_categories_parent_id->fields['parent_id']);
?>
<div id="categoryImgListing" class="categoryImg"><?php echo zen_image(DIR_WS_IMAGES . $catgories_parent_image, '', CATEGORY_ICON_IMAGE_WIDTH, CATEGORY_ICON_IMAGE_HEIGHT); ?></div>
<?php
// categories_image
if ($categories_image = zen_get_categories_image($current_category_id)) {
?>
<div id="categoryImgListing" class="categoryImg"><?php echo zen_image(DIR_WS_IMAGES . $categories_image, '', CATEGORY_ICON_IMAGE_WIDTH, CATEGORY_ICON_IMAGE_HEIGHT); ?></div>
<?php
}
} // categories_image
?>
See if that helps get you started ...