You will need to do some heavy keyframing, or, better yet, motion tracking. This requires software such as FXhome's Lab range of products (found here
) or a good piece of editing software, such as Final Cut (Mac) or Vegas (Windows) to do keyframing. For motion tracking, you'd have to shell out a bit more for a program like After Effects.
Keyframing would involve moving the picture of the head to match the body at specific intervals - usually every couple of frames. This results in anywhere from 5-15 keyframes per second of footage, depending on how accurate you want to make it.
Don't expect your footage to look realistic, though! Doing the effect properly involves a decent amount of time and skill with an application capable of motion tracking, like After Effects. And, even then, a still image will not look photorealistic when superimposed over a moving body.