This was discussed elsewhere, but I don't remember where and a seperate thread solves that. There was some discussion about the difficulty of finding copyright-free pictures to use as the THEN component, and I discovred a great site for it, one you can go to and feel good about your tax-dollars at work. Canadians get a free ride. It's the Library of Congress Prnts & Photographs sections. Here is a lnk to the HABS survey pictures that cover a lot of the US
of A. http://lcweb2.loc.gov/pp/hhquery.html
I have not read all the fine print yet but I am pretty sure that MOST of the images here, and I think, ALL the HABS ones are copyright free. FOr those that were not around in 1936, the HABS [Historic American Building Survey] was a New Deal program to document historic structers in the US. Buildings had to be maybe 50 years old then so they are good and ripe now.
Wandering around other collections there is also very productive. Enjoy eeeeeeeeeeeeeeeeeeeeeeeeeek