No, there is no IPN url that you have to add, in fact we recommend that you don’t add any, but if you want to set a IPN url it must be the home page url.
One other think to check is for a failed IPN file called ipn_log.txt inside the image-store plugin directory.
Be sure you are using the right information (email) for your production environment, it will be different from your development environment.
Also do you know how to get to your server access log? if you do, check for POST request from “PayPal IPN” to confirm that the request is reaching your server and nothing is blocking the request.