The best options are transparent GIF images. For optimum results, the height should be 200. You can change the width. Lower values mean more fluid liquid, so waves end quickly. Higher values cause a slower decadence of the waves.
Higher values mean more distance between one drop and another.
If the parameter is set to zero, no rain generates. Values equal to or higher than 1 will generate rain. You can enable more than one effect at the same time.