Also a term in news broadcasting and other related TV prouctions, a superimposed image is one that goes on top of the main screen. A perfect example would be the stock ticker on CNN, or the pictures they put in the upper corners of news broadcasts. Superimposing an image is done with a computer now-a-days, but older and less advanced studios will be able to superimpose by using the special effects switches and controls on their video mixing board, and do block letter announcments with a character generator.
If one is directing a small studio, there is much fun to be had using the special effects switch, the character generator, and the preview screen, or, if you're up to feeling particularly evil, you can flash-superimpose messages of your superiority, or other such nonsense, over the air waves, just make sure your superimposed image is cut onto the screen instead of faded-in if you're going to do that.