Flash photography has existed nearly as long as photography itself. Early plates and films required the subject to sit still for a long time in order to achieve a sufficient exposure, but by producing a powerful flash (originally by igniting magnesium or some such chemical), enough light could be gathered in a fraction of a second. Though flashes are still needed today, the symbol does not resemble old-school flash hardware because it was never particularly distinctive: just a circle or rectangle near the camera itself.
Fortunately, the flash itself is similar enough to a much older and easily recognized phenomenon: lightning. Lightning produces a quick, bright flash much like a camera’s, and everyone knows what it is. Sometimes the familiar zigzag (usually depicted with only one “zag”) is sharpened on both sides to appear vertically symmetrical, and sometimes it appears overlayed on the camera in lieu of a “real” flash. But the lightning bolt is now inseparable from a camera flash, in both software and on physical camera bodies and flash units.