Modify 'Add to cart' button

-- HikaShop version -- : 2.6.0
-- Joomla version -- : 3.4.5
-- PHP version -- : 5.3.29
-- Browser(s) name and version -- : Firefox 41.0.2
-- Error-message(debug-mod must be tuned on) -- : Need to modify "Add to cart" button

Hello,
I would like to modify 'add to cart' button (for hikashop module) to look it like in the attachment. So, I need to put the cart logo to this button.
I've tried to do it using file in /views/product folder, but seems that it is not possible because all I can see is:

<!-- ADD TO CART BUTTON AREA -->
<?php
if($this->params->get('add_to_cart') || $this->params->get('add_to_wishlist')){
$this->setLayout('add_to_cart_listing');
echo $this->loadTemplate();
}?>
<!-- EO ADD TO CART BUTTON AREA -->

I've found source code for this button at administrator/components/com_hikashop/helpers/cart.php, but all my changes will be overwritten after hikashop update.
Also I've tried to insert it using :before css style for this button in order to set icon via css, but without success.
Is any good solution for this?

Actually I think the simplest way to add such icon is just set background for <input>. I was just thinking about using this icon as <i> element and 'font-awesome' font.
Will try to it tis way. No need to write a reply.