Amazingly, many people who are active on social media sites such as Trulia and Zillow have no clue about the laws surrounding copywrite protection. You do not need to apply for a copy write, or put a watermark on photos, to be fully protected.
I would send a letter of "cease and desist" to the violator and demand the photos be removed immediately. If they do no comply, it would be pretty simply to take them to small claims court. I am quite sure that they would learn a valuable lesson with the lost of one claim.
I would encourage anyone wanting to understand more about copy write laws and real estate to visit Lenn Harley's blog on ActiveRain. Lenn has been instrumental in educating many of us in the full realm of copy write. I remain flabbergasted by the number of social media "guru's" who encourage people to "just go to Google and do a cut and paste of a photo". Unless the owner of the photo specifically allows unlimited use of his/her photo this is expressly prohibited.
Ignorance can be bliss...it can also be very costly!