Summary

RECENTLY ADDED FILE: (CurvLid150prc4to3.stl and Curv150prc4to3.stl) This is the 150% size and 4:3 ratio. So for example if you have a picture that is 1600 x 1200 pixels this is the one to use. When using 3dp.rocks set size 150 mm max. I have not tested it yet but it should work fine.
(Litho1pic4to3CurvedSize2.stl) It is a stand holding one lithophane, assuming max size is 100 mm and it is for lithophane created from a picture with a 4 to 3 aspect ratio. Also added Versions with no holes for electric plug. They are NoHoleCurved with either 1 to 1 ratio for 100mm x 100mm lithophane or 4 to 3 ratio.

This is a frame I designed using Tinkercad. It is designed for curved lithophanes made from square pictures. I make the lithophane by going to this website http://3dp.rocks/lithophane/ and use model size of 100 mm with a 3mm frame. When the model is curved then the STL object will have the size of X=21.6mm, Y=94.0mm, Z=100mm and I position it along the Y direction on the bed for better quality. For the frame I recommend infill 25% or greater. (I like 35%).
There is file for a box lid with 1 pic, and also a lid for a frame with 2 pics. They are really interchangeable so you can use whichever one you like better.
There is a hole 8.5mm in diameter needed for a typical power plug of 5.1mm x 2.1mm. I like to use a short LED strip of 6 to 9 LED's with a 12V DC power adapter. This way the entire assembly can be used as a nite-light that uses 1W to 1.5W only.
I have other designs which I should upload soon, one for a simple base to be used with candle light, one that's for flat lithophanes (100mm x 100mm) and one for RGB LED's that has a base large enough to contain a small microcontroller circuit to control the color etc.