This site works pretty similar to the one OU did of aerial photos of cities a while back but for some historic photos from WWII. To change between the two photos either click on the photo to have it transition to the other one or click & hold the button down then move left/right to control the fading in/out.

Image fader (http://interactive.guim.co.uk/embed/2014/apr/image-opacity-slider-master/index.html?ww2-dday)

Cool pictures