The fab, open-source, OpenPlaques website only has six entries for Washington DC:
http://openplaques.org/places/us/areas/washington_dc
(and needs photos for four of them). It would be great if folk at Wikimania could add to them.
Pics can be imported from Wikimedia Commons; notify OpenPlaques via http://openplaques.org/contact or as @OpenPlaques on Twitter.