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.

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.

Of could during your visit FotoFest DC will be happening, so there will be many things photographic happening.

Enjoy your visit.