After printing a few square lithophanes I realized the corners usually got off the printbed.
In order to fix that, here is the non-square (polygonal) version of the lithophane generator.
Special mention to this other customizable lithophane by MichaelAtOz: http://www.thingiverse.com/thing:78719
Original description from Makerbot:
Click "Open in Customizer" to turn the image of your choice into a Lithopane! We used the Mona Lisa, but you could use any photo you want. The Lithopane tool generates a 3D model that looks like a rough terrain map.
Download the file, print it on your MakerBot Desktop 3D Printer, and you've got a picture perfect pane to hang on your window.
Three cheers for the lithopane pioneers on Thingiverse, such as cymon and pp3dpNL! For more information on this old art form, check out the Wikipedia article here: http://en.wikipedia.org/wiki/Lithophane
Original instructions from Makerbot:
Upload an image, check the invert colors checkbox and create a Lithopane. For best results, upload a square image and print using a light colored plastic at 0.2 mm layer height and 1 shell.
Once printed, hang your picture in a window.