Ren'Py Rain FX
Here's a 4-frame animation I used to create rain over my 1024x768 backgrounds in WAG. Looks kind of bodgy in the little preview GIF, but I promise it looks pretty good in-game.
This one is CC-BY; I really don't mind how you use it, as it's just a collection of GIMP filters and a few lines of code made from directions here on LSF. Ren'Py script for declaring the animated image is included, so you can just drop-in and go!