Wow, sorry Taffeta, that is just so... blatant
People like that should be banned from coffee, chocolate and ponies for the Rest of Their Lives.
I suppose you will need to decide what you are most comfortable with going forward: sadly it seems that people are always going to steal images. They will find a way. So do you want to go the route of watermarking everything or not? It is your website and it is ultimately your choice.
I thought that you might perhaps like to know that our (admittedly small) SA group of collectors think highly of your website as we know you do your utmost to be as honest and accurate as possible. So we can rely on the information.