Lithopanes are objects created to let light through to different degrees, allowing you to create a sort of greyscale image that is visible when the plate is placed against a light source. Created by the folks at Makerbot, the lithopanes are completely customizable and you can upload your own images that will then print in about an hour on a standard printer.
You can see the Thingiverse Thing here and use the Customizer, the company’s own interactive API and object modeler, to upload your own photo. They recommend using a light plastic and choosing a square picture. I foolishly heeded neither of those recommendations so I essentially printed a black, opaque ABS coaster when I tried this on my Makerbot. I’m currently printing it out in light green ABS to see what happens.
The Customizer lets you do all sorts of amazing things, including DIY iPhone cases and customizable rings. It’s striking how close to magic a lot of this stuff is these days, especially with with online tools and inexpensive printers. We, in some ways, live in the future.