To make an illusion you need to start with two .svg images and convert them to .dxf for openscad to use.
I used inkscape ( www.inkscape.org ) to create the yes/no images and the better .dxf output ( http://www.bobcookdev.com/inkscape/inkscape-dxf.html ) to export them to .dxf files. (Hint: you might need to click Path-> Object to Path to get correct output.) Inkscape can also convert images to paths for output.
Change the filenames inside the openSCAD file to match your files. Start with outputting just the shape1() and shape2() modules. Adjust the translation and scale factors in each module until the source images sit exactly on the edge of the x and y axes and adjust the scale factors till they are about the same size (i.e. height). I found that the yes and no images I created were way off centre and needed big adjustments. If you don't see any output and can't see any file errors in the console try zooming out, the image might just be far away.
Run the illusion() module to generate the final shape. You may need to hit F6 to get it to render as the fast F5 won't always work here. Adjust the angle and width parameters to taste. If both source files are the same width a 45 degree angle will work, otherwise try other angles till they work.
Finally - clean up the model. I recommend importing the model into another 3D program to get rid of some unnecessary parts. Try and take off most of anything that doubles up. The less you leave the more impressive the illusion will look. This is why its only a generator(ish).
I have produced two output .stl examples, The YES/NO required a bit of manual clean-up to make it look good. I used Blender for this. The Rebel Alliance vs Galactic Empire one is pretty as generated, but printing it well is left as a challenge to the reader. (The source .svg files used for this are available on wikipedia).