OK, good weekend for me, turns out if you layer up several sheets of paper and then hold them up to the light you can get several different shades of colour. Combine this with careful cutting out of specific shapes within those layers and you can build up pictures within the shades. I took this one step further and wrote a program that calculate this all for you and then prints out some images showing you where to cut but the principle is the same.
It came out really well and not as time consuming as most of my projects are. I've got some more ideas for cutting the paper with a laser cutter or building up an image with a 3D printer that I should probably look into so this won't be the last you'll see of lithophanes from me.
I also wrote an instructable if you want to make your own http://www.instructables.com/id/How-to-Make-a-Paper-Lithophane/