Create your own Kindle screensavers

Kindleboards member moxy789 has posted a pictorial on creating Kindle screensaver images.
Using Gimp, which is a free graphics software package, Moxy walks through the steps to create a greyscale image in the right proportions for a Kindle screensaver. His screenshots make this an excellent tutorial if you want your own custom screensavers, but are puzzled about how to create them to best fit your Kindle’s display.
(Note: Making custom screensavers work on your Kindle requires a hack. Here’s how to do it.)