Using Flickr to Teach Used To

I came across Jason Powell's work recently. Images are made by finding old photographs of places, printing them out, and then holding the print up in the modern day location that the original photograph was taken. So far, most of the historical images have been available for free at the Library of Congress.
As a teacher I see it as a wonderful opportunity to practice grammar, there used to be many trees, or as a conversation sparkle on how different the places are nowadays. Students can write comments or try to make one image of their own communities. How would you use it in class?