If you're set on having your screensaver match your Kindle's skin, pretty much the only way to do it is to have a custom skin made, so that you have the image that the front skin was made from. Then it's a fiddly repeat of looking at where the skin cutout for the screen is, cropping and resizing to fit when it doesn't quite match up, as in this example from my Kindle:
You can see that the left slope of Mt. Fuji doesn't quite line up in the lower-right corner of the screensaver image.
When you're working with the original skin image, crop to the 3:4 proportions first, then resize to 600x800; this preserves maximum image resolution.