The links are output by /includes/templates/your_template/common/tpl_header.php. Unless the output constants are used elsewhere, you can rework the defines to be HTML images instead of text. Any that are used elsewhere would have to be hard-coded in tpl_header.php, or new constant names created there and defined in a language file.
PHP Code:
<div id="navMain">
<ul class="back">
<li><?php echo '<a href="' . HTTP_SERVER . DIR_WS_CATALOG . '">'; ?><?php echo HEADER_TITLE_CATALOG; ?></a></li>
<?php if ($_SESSION['customer_id']) { ?>
<li><a href="<?php echo zen_href_link(FILENAME_LOGOFF, '', 'SSL'); ?>"><?php echo HEADER_TITLE_LOGOFF; ?></a></li>
<li><a href="<?php echo zen_href_link(FILENAME_ACCOUNT, '', 'SSL'); ?>"><?php echo HEADER_TITLE_MY_ACCOUNT; ?></a></li>
<?php
} else {
if (STORE_STATUS == '0') {
?>
<li><a href="<?php echo zen_href_link(FILENAME_LOGIN, '', 'SSL'); ?>"><?php echo HEADER_TITLE_LOGIN; ?></a></li>
<?php } } ?>
<?php if ($_SESSION['cart']->count_contents() != 0) { ?>
<li><a href="<?php echo zen_href_link(FILENAME_SHOPPING_CART, '', 'NONSSL'); ?>"><?php echo HEADER_TITLE_CART_CONTENTS; ?></a></li>
<li><a href="<?php echo zen_href_link(FILENAME_CHECKOUT_SHIPPING, '', 'SSL'); ?>"><?php echo HEADER_TITLE_CHECKOUT; ?></a></li>
<?php }?>
</ul>
</div>
HEADER_TITLE_CATALOG
HEADER_TITLE_LOGOFF
HEADER_TITLE_MY_ACCOUNT
HEADER_TITLE_LOGIN
HEADER_TITLE_CART_CONTENTS
HEADER_TITLE_CHECKOUT
are the constants to check. If they are not used elsewhere, you can redefine them as images.