First of all, there is a good chance that picrights embeds their emails with cookies or something like that which notifies them when someone opens an email.
Second, they already know you are “guilty” of using the image, that is why they are contacting you. The only questions are if you had a license (or a defense, such as fair use) or how you will respond, if at all.
If they are truly harassing you, send them an email asking them to not call or email you again. This will likely result in them stopping.