Open up the image you want to add a page curl to, im going to use my car blueprint from a tutorial i recently created.
Duplicate your image layer and add a 2 pixel stroke.
Now goto “layer > merge visable”, your layers should now have gone into one, duplicate your image layer again and goto “edit > transform > warp”, you should then be presented with this.
Take the very bottom right anchor point and drag towards the middle of your image.
Press the enter key on the keyboard once your done. (you might want to have a play with the warp tool as it is a pretty neat tool)
Now add a simple drop shadow to your image layer.