Chantilly is pretty far outside DC when you don't have a car. Perhaps the hotel services have a shuttle bus service to get to a Metro station.
You may wish to start with Govinda Gallery located in Georgetown. http://www.govindagallery.com/
There are many photographic artists displaying and selling their work directly to the public in the Torpedo Factory in Alexandria, Virginia just outside of the DC. http://www.torpedofactory.org/
Of could during your visit FotoFest DC will be happening, so there will be many things photographic happening. http://www.fotoweekdc.org/index.php?...109&Itemid=223
Enjoy your visit.