The Digital Millennium Copyright Act ("DMCA") of 1998 applies to internet service providers and copyright owners when copyright infringement occurs in the digital environment.
This law provides safe harbors from copyright infringement liability specifically for websites hosting IDX Display.
Recently, Zillow was sued for over 8 million dollars for DMCA violation related listing photography that had a copyright.
To qualify for safe harbor and to protect yourself from copyright infringement, there are a few things NAR legal staff say you must do. See the requirements here.
Also, if your company works with photographers, NAR suggests using one of these sample Photo Agreements to help mitigate liability.
Find out more information about the DMCA and copyright information here and here.