While I'm working on a decent fade-in/fade-out mechanism, perhaps involving a user-controlled slider, the panel below should illustrate the general idea. You may have to wait one cycle for the images to load properly; please be patient. If you see no change after 20 or 30 seconds, it may be that your browser settings aren't allowing the underlying script to run - you may add this site to 'trusted sites' (really, I promise - look at the page source if you like) and reload the page. Otherwise it won't work.
This example was constructed by minimally warping one photo to fit the other; I've written a basic tool to do that for me. It means I don't have to be at the precise spot where the original image was taken from. This photo is of the old Quiambaug School, now a private home. The old photo is from 1909, and possibly shows two of the teachers (at some point I may be able to identify these people).