What a great photo. We uncovered this one at the Historical Society of D.C. website. You are looking down Pennsylvania Avenue, the Treasury building is to the right and the sidewalk is full of nicely dressed pedestrians.
A Glimpse Into the Past: Viewing Pennsylvania Avenue in Front of the Treasury Building
The “Historical Society of D.C.” (i.e. http://historydc.org/exhibits/window/19th%20century%20prints.html) link doesn’t work.