This plug-in uses a shaped gradient and multiple layers to make a staggered border of different colored lightning. The image can be from 300x300 to 3000x3000, and the image can have up to 20 frames of lightning. The hues of the lightning gradually change as you go further toward the center of the image.
It's a difficult image to try to describe, so an image of what this script does is attached. This is an 800x600 image, using 5 frames. Using one frame will give a lightning border that can be used for any sized image.
The script works with GIMP Version 2.6. Enjoy!