Have you Googled āLithophaneā?
Try looking at https://www.youtube.com/watch?v=WYDYGDIPteo which shows how to cut a photo into something and to then back-light it. Very cool techniqueā¦
Seems that the original software is no longer available. I found another software at http://www.halftone.com.br/ which is not free however seems like it does a good job. If anyone knows of a free version that they have tried please let us know. Thanks