Step 1: Open the picture in which you want to have this effect.

Step 2: Unlock the background layer, this is done by double clicking on the lock in the layer palette as shown below and then rename the new layer as original as shown in the dialog box below.

Step 3: Now select the Rectangular Marquee Tool.

Step 4: Now select that portion of the photograph where you want the page curl effect.

Step 5: Select the Gradient tool and then draw the black to white gradient in the selection you have made. I have selected from left top corner to bottom right corner.

Step 6: Click on the Deselect option in the Select menu.

Step 7: Now select the Wrap option in the Edit Menu.

Step 8: Drag the bottom right corner towards the centre to have the curl effect.

The Final Image is given below: