Detect a camera flash? Light sensor, photoresistor?
A photo-resistor would not be recommended to detect a camera flash accurately.  For example, the Jameco Part #202403 (Part PDF) has a response time of 50 to 75 milliseconds.  Photo-resistors are more suitable for general light levels, for instance, like how bright it is in a room.  On the other hand, photo-transistors are suitable for very fast reaction times.  The Jameco Part #2129385 (Part PDF) has a response time of 15 microseconds, which is more than 1000 times faster.  A photo-resistor will be able to respond to the light change, but it will be nowhere as sensitive.As with any product, however, search for the product PDF before buying it.