I've made this for valentines day.
Used photos taken by my iPhone 6 front camera.
So every picture taken by your iPhone 6 front camera fits immediately.
Otherwise you can change the resolution of any other picture.
Designed the box in Autodesk Inventor professional 2017.
Made the lithophanes 3mm thick, execpt the grey one is 2mm.
The lithophanes are online made at: http://3dp.rocks/lithophane/
All sliced with the latest SLIC3R.
No supports used, 0.1mm layers, 0.4mm nozzle and a 5mm brim.
drilled holes in the 4 posts of the lid and in the bottom.
And inserted metal rods in it for a clean closing of the box.
Feel free to ask questions about it