Sounds like you want a "form button", as is used in the sidebox. That would require a little more effort. I'd recommend simply "styling" the link as desired. For example:
Code:
<?php
// (un): start wishlist link
echo '<a href="' . zen_href_link(UN_FILENAME_WISHLIST, zen_get_all_get_params(array('action', 'products_id')) . 'products_id='.(int)$_GET['products_id'].'&action=un_add_wishlist') . '" style="background:#ccc; padding: 5px;">' . UN_TEXT_ADD_WISHLIST . '</a>';
// (un): end wishlist link
?>
Notice I added some CSS to the <a> element via the "style" attribute. You can use inline style definitions like this or external style sheets to control the appearance of any link. You might want to check out the following site for more info:
http://www.w3schools.com/css/css_intro.asp
Another option is to create an image. Replace "UN_TEXT_ADD_WISHLIST" with your image tag and you have complete control over the appearance of the "link".