How to superimpose still image in video

I am trying to superimpose a still image into video with a lot of movement. Is there anyway to do that without having to place the image in every frame? I have a video of a dancer and wanted to replace the head with another person's head.

what software do you have available?

I have Final Cut Express and imovie

in final cut express you can use the keyframing feature, it will take longer then motion tracking if you had motion or after effects, but it can be done in fce. it will be a long process but ever time the head moves in a straight line (ever few frames or so) just move the image you super imposed to follow. The keyframe button looks like a diamond.. its on the viewer and canvas windows but you'll need to use the one on the canvas.

I have done that a little and you are right it takes a looong time and it's difficult to get the head exact with the constant motion. Is there a better program for this?

motion tracking both of the eyes of the real dancer to the eyes of the cut out that is super imposed and using the tracking data to scale and rotate based on the eyes closeness to each other and the angle they are at. You would need a program like after effects or motion to do so.