All of the links mentioned in this tutorial can be viewed here:
http://gimpedtutorials.com/index.php/how-to-use-gimp-2-8-to-make-the-rainy-day-image/
This Gimp Video Tutorial will teach you how to use gimp 2.8 to make the rainy day wallpaper from scratch using layers, layer masks, and a few downloadable brushes.