First create a spotlight. Change Decay Rate to Quadratic. This will give us a more realistic falloff to our light. To compensate for this falloff, increase the intensity to 50000. Increase the cone angle to around 110 and the dropoff to 1. Turn on Use Raytrace Shadows on the spotlight and slightly increase the light radius to 0.010. This will give us a soft edge to our raytraced shadows.
Create a single polygon plane and position it behind the spotlight. Scale the shape of it to make a rectangular shape.
Increase the incandescence to a high white value. You might want to increase the incandescence value above one in the colour chooser. This will give you an extra bright value for your flash that will show up well in the reflections in your scene..
Group the spotlight with the polygon reflector plane and scale them down to about 0.01. By doing this we are scaling them to the approximate size of a real camera flash. Use a transformation tool to better see the position of our newly scaled flash light