When I tried out pp3dpNL's Mona Lisa Lithophane I printed it on its side and what I ended up with (on the left) was disappointing. Not only did it take forever to slice and cracked while printing (darn ABS) the final result just didn't look very good. The one on the right above was printed flat on the build surface using a STL with 1/10th the polygons and the result was much better. I'm still not sure what fixed it, the orientation or the polygon count, but I suspect it was the former since skenforge has some known problems with fill in sharp corners. Both were printed with 0 extra shells and 100% infill.
Keep in mind what you're seeing only looks the way it does because it has a bright light behind it. Without that light the lithophanes are recognizable but... wrong (see right).
Now that I know how, tho, I think I'll be able to reproduce this process for other pictures. Could be a new angle on the customized 3D things market.