I recently created a Lithophane of Luke Hemmings from 5SOS for my daughter by 3D printing an image converted into a height-map.
If you want to do this yourself, find a contrasty image, convert it to black and white and then convert it to an STL using the following site: http://3dp.rocks/lithophane/.
Be sure to set the image settings to Positive Image and set the height in the Model settings to 4mm (material translucency dependent) and set the Vectors per pixel to 5. Add an optional border (mine was 5mm) and a stand to make it easier to print.
Once you have the STL you must print it standing up. If lying down the grayscale range will be compressed to only a few shades because of the layer height limitation. Print no faster than 3000 mm/min (50 mm/sec) and use no in-fill and around 6 shells to make it nearly solid. My printer printed the image fine without any supports.
If you want to know more about Lithophanes then see the Wikipedia article here: https://en.wikipedia.org/wiki/Lithophane
They were originally made out of porcelain and have been around for hundreds of years.