This script makes a ring system that you can add to a picture of a planet, and was adapted from a Photoshop script. This script is updated for GIMP v2.6
The ring system can be any size from 300x300 to 3000x3000, and can be tilted to any angle. The whirl-and-pinch portion of the script is time consuming, but using at least 8 iterations make the rings look very nice. The thickness of the rings can be controlled using the inner and outer ring ratio values in the script. The closer these two values are together, the narrower the ring.
After the rings are rotated, the image is autocropped to make it as small as possible. There is an option to automatically colorize the rings, but I usually find that it's best if you do that manually after the script has finished running.
If you do paste this into a picture of a planet, set the layer mode to Screen. You can remove the portion of the ring that would be behind the planet by making layer with the planet active, and choosing Alpha to Selection. Then activate the layer that contains the ring and erase the portion of the ring in back of the planet using either the eraser tool (simple but time consuming) or a marquee tool set to "Intersect with current selection" and cutting and pasting.
I've also attached a small picture of what the rings look like around a blue planet. Hope you like this one...