The site is Flux Machine, http://fluxmachine.tumblr.com, by Kevin Weir.
Weir takes scans of old photographs from the Library of Congress and modifies them into animated GIFs. Most of them are spooky, some are a bit gruesome, but all of them are really neat. I think “Priest, Valamo Monastery, Karelia, Russia” is my favorite, but there’s lots of others I like too.
Can’t remember where I first found the link, maybe on Wired‘s page.