I think may have been called Photomontage or Photomosaic. I used it a few times but there didn't seem any practical use for what it did. I wasn't using it for porn though, I think it had a default folder with images.
IIRC that was the era of Kai's Power Tools/Goo and Poser and other quite interesting graphic tools.
I realize this is zero help looking for a free one though, but that technique is probably relatively easy to do these days.
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