Greetings, all.

I'm trying to make some changes to my template, notable changing out the button images for the various functions.

Now, to start, I'm trying to add a new "add to cart" button.

I have located the following code:

Code:
<!--bof Add to Cart Box -->
<?php
if (CUSTOMERS_APPROVAL == 3 and TEXT_LOGIN_FOR_PRICE_BUTTON_REPLACE_SHOWROOM == '') {
  // do nothing
} else {
?>
            <?php
    $display_qty = (($flag_show_product_info_in_cart_qty == 1 and $_SESSION['cart']->in_cart($_GET['products_id'])) ? '<p>' . PRODUCTS_ORDER_QTY_TEXT_IN_CART . $_SESSION['cart']->get_quantity($_GET['products_id']) . '</p>' : '');
            if ($products_qty_box_status == 0 or $products_quantity_order_max== 1) {
              // hide the quantity box and default to 1
              $the_button = '<input type="hidden" name="cart_quantity" value="1" />' . zen_draw_hidden_field('products_id', (int)$_GET['products_id']) . zen_image_submit(BUTTON_IMAGE_IN_CART, BUTTON_IN_CART_ALT);
            } else {
              // show the quantity box
    $the_button = PRODUCTS_ORDER_QTY_TEXT . '<input type="text" name="cart_quantity" value="' . (zen_get_buy_now_qty($_GET['products_id'])) . '" maxlength="6" size="4" /><br />' . zen_get_products_quantity_min_units_display((int)$_GET['products_id']) . '<br />' . zen_draw_hidden_field('products_id', (int)$_GET['products_id']) . zen_image_submit(BUTTON_IMAGE_IN_CART, BUTTON_IN_CART_ALT);
            }
    $display_button = zen_get_buy_now_button($_GET['products_id'], $the_button);
  ?>
  <?php if ($display_qty != '' or $display_button != '') { ?>
    <div id="cartAdd">
    <?php
      echo $display_qty;
      echo $display_button;
            ?>
          </div>
  <?php } // display qty and button ?>
<?php } // CUSTOMERS_APPROVAL == 3 ?>
<!--eof Add to Cart Box-->
My question, is... SOMEthing in there is setting the "add to cart" button to: /includes/templates/template_default/buttons/english/button_in_cart.gif

Where is this particular filename designated for the various buttons? Is there a config file that sets the filenames somewhere (I've looked all over for it)?

(I don't want to just overwrite the existing images, since I want to use PNG/JPG for some additional image treatment).

In a nutshell, I want to make it call button_in_cart.png instead of button_in_cart.gif

Thanks!