Amazon suggests that there is a way to do it:
"Amazon.com Customer Service does not support or provide instructions for adding custom images to your Kindle to be used as screen savers. However, should you choose to do this on your own it will not void your warranty."
http://www.amazon.com/Amazon-com-Cus...x2HTAOBPKHJ0P8
Now, one of you geniuses needs to find out how.