OK... this may point you in the right direction:
After creating an appropriate image for the button, and defining the button, my edits are in:
includes/modules/MY-TEMPLATE/product_listing.php
(Naturally, the CORE file is in /modules/product_listing.php)
The following lines are from about line 179 - 180.
// qty box with add to cart button
if (PRODUCT_LIST_PRICE_BUY_NOW == '2' && $listing->fields['products_qty_box_status'] != 0) {
$lc_button= zen_draw_form('cart_quantity', zen_href_link(zen_get_info_page($listing->fields['products_id']), zen_get_all_get_params(array('action')) . 'action=add_product&products_id=' . $listing->fields['products_id']), 'post', 'enctype="multipart/form-data"') . '<input type="text" name="cart_quantity" value="' . (zen_get_buy_now_qty($listing->fields['products_id'])) . '" maxlength="6" size="4" /><br />' . zen_draw_hidden_field('products_id', $listing->fields['products_id']) . zen_image_submit(BUTTON_IMAGE_IN_CART, BUTTON_IN_CART_ALT) . '</form>';
} else {
$lc_button = '<a href="' . zen_href_link($_GET['main_page'], zen_get_all_get_params(array('action')) . 'action=buy_now&products_id=' . $listing->fields['products_id']) . '">' . zen_image_button(BUTTON_IMAGE_BUY_NOW, BUTTON_BUY_NOW_ALT, 'class="listingBuyNowButton"') . '</a>';
}
}
}
if (!zen_has_product_attributes($listing->fields['products_id'])) {
$lc_text .= '<br />' . '<a href="' . zen_href_link(zen_get_info_page($listing->fields['products_id']), 'cPath=' . (($_GET['manufacturers_id'] > 0 and $_GET['filter_id']) > 0 ? zen_get_generated_category_path_rev($_GET['filter_id']) : ($_GET['cPath'] > 0 ? $_GET['cPath'] : zen_get_generated_category_path_rev($listing->fields['master_categories_id']))) . '&products_id=' . $listing->fields['products_id']) . '">' . zen_image_button(BUTTON_IMAGE_MOREINFO, BUTTON_MOREINFO_ALT) . '</a>';
}
$the_button = $lc_button;
$products_link = '<a href="' . zen_href_link(zen_get_info_page($listing->fields['products_id']), 'cPath=' . ( ($_GET['manufacturers_id'] > 0 and $_GET['filter_id']) > 0 ? zen_get_generated_category_path_rev($_GET['filter_id']) : $_GET['cPath'] > 0 ? zen_get_generated_category_path_rev($_GET['cPath']) : zen_get_generated_category_path_rev($listing->fields['master_categories_id'])) . '&products_id=' . $listing->fields['products_id']) . '">' . zen_image_button(BUTTON_IMAGE_MOREINFO, BUTTON_MOREINFO_ALT) . '</a>';
COMPARE my code with the same section in the core file to see my alterations - chiefly
$the_button = $lc_button;
$products_link = '<a href="' . zen_href_link(zen_get_info_page($listing->fields['products_id']), 'cPath=' . ( ($_GET['manufacturers_id'] > 0 and $_GET['filter_id']) > 0 ? zen_get_generated_category_path_rev($_GET['filter_id']) : $_GET['cPath'] > 0 ? zen_get_generated_category_path_rev($_GET['cPath']) : zen_get_generated_category_path_rev($listing->fields['master_categories_id'])) . '&products_id=' . $listing->fields['products_id']) . '">' . zen_image_button(BUTTON_IMAGE_MOREINFO, BUTTON_MOREINFO_ALT) . '</a>';