Hi Deb, I'm not exactly sure of the difference, but from reading the dictionary definitions i would guess that superimpose might be used when you mean to place one image on top of another-- In the photograph, his head was superimposed on the other person's body . Superpose can mean place above, although i'm not sure how you would use it in a sentence. It also has a mathematical meaning of placing two curves or graphs over each other so they overlap exactly.
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