You're never going to get great results for a complex lithophane on any slicer. For simple ones you can get decent results, but the one in your picture is tough for a 3D printer that uses FFF technology.
I have tried doing lithophanes of people using several different slicers and they all came out pretty ugly. There were too many line gaps and strange line patterns, making the faces look like zombies. If I could simplify the lithophanes, they would look better, but then facial features would get lost.
As long as you're not expecting great results, I would try to increase the number of Top Solid Layers, or make your infill 100% solid so that the complex lithophane area has more support. Even if you fix some of the line gaps, you can't fix them all, and it is still going to be pretty ugly. I gave up doing lithophanes for anything complex, unless a rough look is desired. A lithophane of large text looks good. A lithophane of a simple cartoon face looks good. But a lithophane generated from a complex photo tends to look very ugly.