“A lithophane (French: lithophanie) is an etched or molded artwork in thin very translucent porcelain that can only be seen clearly when back lit with a light source.”
3D-printed plastic works fine, too. There is an excellent post on the RepRapPro blog describing the whole process from start to finish. This was something we just had to try ourselves, of course.
There are a number of steps to follow but it is not particularly difficult. Images with high contrast work best; cartoons and woodcuts are ideal. This Sonic the Hedgehog lithophane was printed in PLA with a 0.35 mm nozzle and a layer height of 0.1 mm (base layer 0.26 mm).
Our tribute to M.C. Escher‘s Dag en Nacht (Day and Night). This is in white PLA, same settings. The result is quite good, if we may say so ourselves, especially considering that the original was printed in black and gray ink, not just black.