The file you reference is set to a define in includes\languages\english\button_names.php
Code:
define('BUTTON_IMAGE_SOLD_OUT_SMALL','button_sold_out_sm.gif');
And you'll find that define used in the zen_get_buy_now_button() function in includes\functions\functions_general.php
Code:
case ($button_check->fields['products_quantity'] <= 0 and SHOW_PRODUCTS_SOLD_OUT_IMAGE == '1'):
if ($_GET['main_page'] == zen_get_info_page($product_id)) {
$return_button = zen_image_button(BUTTON_IMAGE_SOLD_OUT, BUTTON_SOLD_OUT_ALT);
} else {
$return_button = zen_image_button(BUTTON_IMAGE_SOLD_OUT_SMALL, BUTTON_SOLD_OUT_SMALL_ALT);
}
break;
These types of questions are easy to track down if you use a text editor that can search contents of a file. I highly recommend Notepad++ if you code in Windows.