Black and white photographs can be printed onto almost any surface- not just paper.
Generally, the process involves painting a special silver gelatin emulsion onto the
surface, in the dark room. The surface is exposed to film, taken through developer
and fixer, and dried.

Here’s a description of how I created the photos that were printed onto
clamshells:

1. Clean the shell first with water/soap, then with bleach in order to whiten the
   surface.
2. Coat the surface with gelatin [1 package plus 1 1/2 cups water, dissolved]. Drain and dry.
3. In the dark room, warm and solubilize the silver gelatin- I use MACO variable
   contrast [Photographer's Formulary].
4. Paint the liquid on the shell, in one direction. Stand upright. Air dry or
   carefully blow dry.
5. Repeat [4] but paint at right angles. Stand, dry, and store in light tight bag [I
   use old print paper bags].
6. Expose to film in enlarger. Note that a shell of similar size but not treated
   should be used to guage the correct enlarger settings. Usually I run a “test shell”
   with different exposure times in order to get the correct exposure.
7. Carry through developer [Dektol], 2 fixer baths, and water. Dry thoroughly.
8. Spray with clear Krylon acrylic spray [obtained at any hardware store], dry.

Enjoy your finished clam shell, and feel free to ask me any questions about this technique. I hope you find it useful.