For the best 3D lithophane results, start with a well-exposed image that has balanced contrast. 

Images with large dark areas can get muddy and lose detail. This image has no detail in the dark areas. The lithophane will be dull.

The same image as above but with more detail in the shadows and less contrast will print well.

Subtle midtones and clear separation between light and dark areas help make a great print. 

Converted to B&W, this image loses its excitment. The loss of detail will cause large dark areas on the print and it will feel muddy.

Enhancing exposure and lightening the dark areas in your image will make for a great lithophane