Funny how you can look for ages, and then as soon as you post a question you find it :o)

I found the following worked okay for me:

includes>templates>template_default>templates>tpl_index_products_list.php

Find:
<div id="categoryImgListing" class="categoryImg"><?php echo zen_image(DIR_WS_IMAGES . $categories_image, '', CATEGORY_ICON_IMAGE_WIDTH, CATEGORY_ICON_IMAGE_HEIGHT); ?></div>

Amend to:
<!--
includes>templates>template_default>templates>tpl_index_products_list.php
<div id="categoryImgListing" class="categoryImg"><?php echo zen_image(DIR_WS_IMAGES . $categories_image, '', CATEGORY_ICON_IMAGE_WIDTH, CATEGORY_ICON_IMAGE_HEIGHT); ?></div>
-->