This is a simple remix of a great tool from madpenguin. The angle of the "tabs" keeps the filament from sagging when trying to bridge.
To use this, put your lithophane on the bed in your slicer, standing vertical, along the Y axis. Put a side support right along each outside edge. (see pics) Very close, but not quite touching the lithophane frame. When sliced, you should see each has it's own walls (perimeters)
To scale, scale the supports to the size of your lithophane, in the Z axis only, (turn off uniform scaling)
If you print a lot of lithos the same size, once you have your supports the correct size and orientation, export them both out together into a new .stl. This saves time moving and orienting each time you print a litho.
I use multiple processes in s3d to slice the supports at a higher layer height from what the litho is sliced at. This will save some printing time. I also suggest using skirt and raft. Once dialed in, everything prints steadily, and raft and supports break away cleanly.
As a bonus, it also acts as a stand until you are ready for a backlight.