Check out Oak Cliff before and after by Justin Cozart:
Oak Cliff (my neighborhood of residence), is one of Dallas’ most interesting neighborhoods and definitely the least idiomatically Dallas. Do check out the fine imagery.
(I mixed that image up all by my big self, aren’t you proud?)
Adam Keys is typing 
Did you mix-up that image via a layer-mask, or did you go for another technique?
I pasted the two images into OmniGraffle and then put a triangle shape on top of each image. I then did a shape subtract to get just the part of the image I wanted. Then I had to line up the images, as they perfectly in sync. As you might notice, the top corner isn’t exactly split between then and now.
If there’s a way to do this in something like Acorn, I’d love to hear about it.
In short: fear my ability to abuse OmniGraffle!