Mathematica 11 has a new feature can convert graphical plots to printable files. Inverted a photo, made an intensity plot of it and converted it to an STL. Needed to be stretched a bit vertically to be thick enough to print. As you can see, the print looks a bit of a mess, but when you backlight it....

http://3dp.rocks/lithophane/ it's prone to crash but has a neater output-though the Mathematica 11 lithophane looks more organic/artistic!

Lithophane. I had no idea that there was a name for it. I'll try printing the one from the website. This is definitely not what the function in Mathematica was intended for!

When put in the same size etc the website and Mathematica versions come out pretty close although the Mathematica version had a bit better contrast. Mathematica is incredibly slow though. It generates an STL file that is 3 x the size. Presumably a mesh that is finer than it needs to be. The website version didn't like Safari but seemed to work OK on Firefox. I'll use the web version if I want to do any more.

just printed a lithophane myself and they are fun to make, but i used slightly too opaque filament so it doesn't show up that great