The lithophane program I used are in the link above. For pictures there is all kind of software out to help remove the background as it make it stand out better. Sometimes it is a bit tedious to try to do that because you have use a cut out tool and get close to the edge of the image. This step is mor like picture editing before it is use in the lithophane.
The video above helps to understand the lithophane. Most of the small one I created was 4mm thick but the image in the program are set min 0.8mm and max 3.2mm that is for the difference between light and dark of the image.
In the program you upload that image you removed the background then Edit, Image Editor change the Brightness and Contrast as needed,
You can also Image Transformations crop the image to help centering it.
Shape options, Shape you pick what you want to creat, I would recommend trying flat first as for the curve one there is a bit to adjusting the image so it dont look squishy.
You can change the size there too but for now keep the default value I stated above for the image.
Like for the Heart shape for example there is also Attributes where you can select Enable Hook and size it, keep in mind about how its printed, I printed those Upside down still vertical.
Down the bottom left Image Options help with the position and size of the pic in the frame.
Printing is better with low layer lines and in line with the Y direction so that the X move is minimal because of the thickness.
I made a few mistake myself when I forget about NO INfill so you cam say 100% or 0% with a line count on shell at 5. I also made mistake in playing with the Depth Min and Max when I didnt understand what it was for. I include a round shade that the infill was on and image was squish because it tried to wrap it around and a night light cover that didnt turn out well because of what I did to the min and max depth.