Of course, the thing is that eInk doesn’t require a screensaver.
Nevertheless, the Kindle, Nook, and Sony Reader can display an image when they go into sleep mode.
In the instance of the Kindle, it now requires a hack; that prior option was taken away probably due to Customer Support calls. See the link at the end of this post for the minor hack required.
And you’ll notice I haven’t mentioned Kobo. It has its own scheme, sometimes displaying the cover of the book being read.
Here are two resources I bumped into on Flickr today:
Here’s an example:
All are 600 x 800, ready to be downloaded and installed.
HowTo: Sony Reader Screensavers
Add Your Own Picture to the Kindle Screensaver