You can't vary the image points themselves, but you could create multiple image points and have the bullet spawn from a random one.
Alternatively, use the system>create object function to spawn the bullets at "spawner.ImagePointX(0)+random(-10,10)), spawner.ImagePointY(0)+random(-10,10)" (or however much of a random spread you want).
Altering speed, as you said, is also a good way, but that depends entirely on your gameplay dynamics, and generally bullets move at the same speed when fired from the same gun, anyway.