Replacing head to create funny effects

Start by opening and image of a boy.

Clone the face area using clone stamp tool.
Insert an image of a monkey. We will replace boy's face with monkey's.

Knock out the background by creating selection using lasso tool.
Click add vector mask button from the bottom of the layer palette.
With black as a foreground color, select a brush with feathered brush setting and apply on unwanted area. That area will be masked out.
Insert a cutout of a hat and place it over the head of monkey.

As we knockout the background of monkey similarly knockout the background of boy layer. Insert any interesting background and move the layer below all the layers in layer order in layer palette.