This is a short tutorial that shows how you can easily create an effect like a swirl like you see them on spinners on aircraft. http://www.simmerspaintshop.com/forums/gimp/1382-making-prop-swirls-gi...
Layers offer you great possabilities to work. You just need to understand how they work and what tricks you can do with them http://www.simmerspaintshop.com/forums/gimp/512-using-manipulating-lay...
Start from a photo or drawing, transform it in animation bu creating multiple layers and and a fake rain using motion blur. http://howto.nicubunu.ro/gimp_rain_animation/