To make the caustics, first I placed a sunlight (infinite) in the scene with intensity 1600% (Yes you read that correctly, 1600%)

To get the shadows accurate you should set the Shadow Blur Radius to .5, but you can manipulate that for "art" if you want.

I grabbed the black-and-white image from this page: https://www.filterforge.com/forum/read.php?TID=12886

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)