First, choose (or draw) the digital image that you wish to put on the picture. Higher quality images will obviously look more realistic. Something with a white background works best.
I chose a piece of artwork gleaned from Mark Haddon's art blog
of a toilet monster, something I deemed suitably shocking- yet not entirely unbelievable- to my mother and father.
Next, take a picture of the body part you wish to "modify" in your image editor.