This is an example of a very easy, yet very effective, lightning effect. Step 1- Take the frame before the lightning is supposed to flash. Make it a png image and bring it into photoshop. Step2- In Photoshop; duplicate the layer. Step3- Use the Image-Adjustments-Threshold, to change the duplicated layer into a black and white image. Step4- You have two options. You can either duplicate the black and white layer and simply do Image-Adjustments-Invert, or you can duplicate the color frame again but adjust the threshold to make it look different than the previous threshold adjusted layer. Step5- Again duplicate the color layer and make a third threshold adjusted layer that is different than the other two. Step6- Export each of the three black and white layers as separate png files, import the png files into whatever program you are using, Flash, ToonBoom, or After Effects, insert the png's into frames, on two's or ones, or a combination, whatever works for the scene.
It ends up looking like THIS EXAMPLE from my earlier post.