General Photography:
* Photo (the most basic form)
* Photography
* Photographer
* Photogenic
* Photographic
Science & Technology:
* Photon (a particle of light)
* Photoreceptor (a cell that detects light)
* Photosynthesis (process plants use to convert light into energy)
* Photovoltaic (relating to the conversion of light into electricity)
* Photoluminescence (emission of light after absorbing light)
Other:
* Photocopy
* Photostat
* Photomontage
* Photosensitive
* Photoplay (an early term for movies)
Let me know if you'd like more examples, or want to explore words with a specific meaning related to "photo"!