http://interactive.guim.co.uk/embed/2014/apr/image-opacity-slider-master/index.html?ww2-dday (http://interactive.guim.co.uk/embed/2014/apr/image-opacity-slider-master/index.html?ww2-dday)
Click on the right side of the pic, right-click and hold, and the scene will shift back and forth in time as you slowly drag the arrow across the pic.