Images may be superimposed by blocking some rays in front of the lens, behind
the lens, or at the film plane. However, the results won't be the same.
Blocking rays near the lens causes a gradual dimming of the image so
the superimposed images fade into each other quite gradually. Blocking at
the film plane gives a sharp division, almost like two separate pictures.
One thing that may be causing folks to be confused about all this is that
in class we tend to sketch lenses that are as big as our object and immage.
In photograpy the lenses are usually smaller than either the object or the
image and we have objects and images that are pretty far off axis. It is the
obliquity of the rays that allows a portion of an image to be blocked (mostly)
by placing a card (or hand) in front of the lens.
If nothing else we've certainly established some common misconceptions.