To get the best results with your lithophane you need to use the best settings. You will want to print your lithophane slowly, so your print speed settings should be between 40 and 60 mm per second. The slower you print, the more likely you will get better results without losing a lot of detail.
For thickness settings, you should start with a maximum size of 150 mm and a thickness of 3 mm with 0 sets for the border. Using the thinnest layer settings makes it possible to choose the smaller width of your image.