Unfortunately, I found it hard to find pictures that were already formatted for the grey-scale of the screen and the Nook's 600x800 active resolution (the Nook Simple Touch has a physical display with a 4.8 inch height, 3.6 inch width, and 6 inch diagonal), so I took some of my desktop backgrounds, in addition to some new pictures I found, and formatted them in Photoshop. I think they look pretty good as Nook screensavers, so I figured I would share them with the internet too. These should also work well as Kindle screensavers, or any other E-Reader out there.
If you want to bulk download all of these, I suggest going to the Picasa Album I have them posted under and clicking "Download to Picasa" under the "Actions" menu. Of course, you will need Picasa to do this, but really, you should have Picasa anyway; awesome program for sorting and viewing pictures on your computer.
I also have the pictures embedded below, after the page break. Click on the thumbnails for the full resolution versions and a Lightbox slideshow view.
11/25/2011 Edit: Fixed broken Picasa Links and Embedded Images