Combination of photos from past and from present can be very interesting way to create great artworks.
Sergey Larenkov is Russian photographer who managed to create few of such combined photographs. He found some photos of places in Russia and rest of the world from World War II and he took photos of same building again, from same angle. Then, he combined old ones with new ones to get that "ghost effect".

I have to admit this is very cool idea and very impressive work where you can see people from WWII and today's people standing next to each other.