If you haven't figured this out already, Lithophanes are one of coolest things you can print with your 3D Printer. However, getting the proper settings can be confusing and very frustrating.
Tool to convert Images for 3D printing at lithophanes: 3dp rocks
A good starting point with your print settings.
Maximum Thickness: 3mm
Minimum Thinness: .6mm or .2 plus your nozzle size
Negative: If you have the option print with a negative setting
Perimeter Speeds: 35mm/s - 45mm/s
Print Along the Y direction