Checking for Stolen Images on Site

Site URL: https://www.uniquegranitecorp.com/

Hello, just today we received an online form submission from a person who says that they are an experienced photographer/illustrator and they claim that we are using some of their pictures on our site illegally. They ask that the pictures be removed within the next couple of days or they will sue us over it. They supplied a Google link to their evidence which we were wary of opening at first due to safety concerns, but after checking we decided to go ahead and open the link. Unfortunately , the link would not actually open and it seems that it is just not working overall. I emailed the person to the email they supplied and I received and error with my email failing to be sent. I checked the persons name hoping that maybe some of their work would appear, but had no such luck either.  

As far as we can recall, the only images on our site were either stock photos obtain directly through Squarespace or personal images that we uploaded ourselves. Our question would be whether there was a way to check for stolen images on our site to ensure that we are not violating copyright or what we should do in this situation. We have received plenty of other spam and offers so we were also left wondering whether this could just be a scam as well. Any advice would be greatly appreciated. 

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