Inserting a still image into video footage - how to?

I have a video scene that has a picture frame hanging on a wall in it. I would like to replace the content of the picture frame with a different image. The camera is not moving, so the frame's position on screen is fixed. The new image would have to be pasted just over that part. (There is movement in the background, so I can't just use a giant still image for the entire shot.)

Any idea how I could do that? Preferably in iMovie, or another application that I don't need to spend money on?

Hmm...not sure if that is exactly what I am trying to do...it's tricky to explain.

I don't want to freeze the entire frame, cause there is movement on the left side of the screen. I just want to have a still image "hovering" over one part of the screen in a fixed position, while the scene is in motion.

I don't have iLife 09...

However, I found a way to do it in 08. You can overlay an image while the video is running. By giving that image a transparent background, you can have it cover only part of the screen, with the video running in the background.

But in another unexpected turn of events, I just ordered Adobe's CS4 Production Premium (Student), which contains Premiere. That should probably do the job!