This security fix is regarded as low risk because hacker would need to have admin access.
The popup page for additional images e.g. index.php?main_page=popup_image_additional accepts a GET parameter for products_image_large_additional.
Using a crafted URL an attacker can determine (via the html returned) whether a specific file exists on the server. This flaw does not indicate the attacker can ACCESS the file, just find out if it exists on the server.
If you would like to install this fix yourself, instructions are included on the official Zen Cart website. You may also visit the site to get technical details of this risk