You can create a much more realistic lightning effect by.
First drawing a black and white gradient accross the middle of the image, then putting a Difference Clouds filter on it, then press Ctrl+i to invert the colors. Then press Ctrl+l to bring up the levels dialog, drag the middle slider right until a nice effect appears. Then if you want some color press Ctrl+u and tick the colorize box, drag the sliders around to make some pretty colors. Presto a nice lightning effect.
