It quick glance it looks like blended images using gradient masks.
You start off with two images on seperate layers.
Select the top layer and position it where you like it.
Change your foreground color to black and your background to white in the tools palette.
Click the layer mask icon on the bottom of the layers palette.
Select your gradient tool and make sure it is set to foreground to background.
Drag over your top layer torwards the back layer.
You can redrag over again until you get what you like. Play around. Have fun.