The iMovie video effect "Flash" makes it appear that a camera flash goes off in the middle of the video clip.
iMovie Video Effects - Flash
Effect In, Effect Out:To apply the effect to only part of the video clip, move the sliders to the places/times when you want the effect to begin and end. Leave them both set to 0:00 to apply the effect to the entire clip.
Count: Move this slider to adjust the number of times the flash goes off during the course of the clip. At One you'll see one flash, at Max you'll see multiple, consecutive flashes.
Brightness: This slider adjusts the brightness of the flash. At Max the flash will turn the screen completely white, at Min the flash will only brighten the screen slightly.
Speed: This slider can be used to adjust the length of the flash.