Step A.-
Raindrops colour
You can change the raindrops colour by changing the colour on the first param name
Instead of gray you could use black or white depending on how light or dark your picture is.
Step B.- Angle of rain
You can change the angle of the rain by changing the value
0 = straight down ~ 1 = left to right, etc
Step C.- Changing the Width and Height of your applet
You need to change the width and height to suit the size of the picture you're using, so please make sure you have the size of your picture correct or it will resize it to the values you're giving it.