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?