I have made lithophanes for years now and am looking for a way to make bas reliefs, that isn't so easy.. I know some Blender script but that is still too hard for me.. I have seen some depth maps in black and white that create nice reliefs when converted as a lithophane.
I have used MeshMixer and VisCAM to edit the mesh some more.
I converted this picture to the Buddha relief you can 3D print: http://pages.cs.wisc.edu/~lizhang/courses/cs766-2008f/projects/phs/students/barnard/results/buddha/depthMap.png
Now only to find an easy way to make depth maps!