You can use your own images for screensavers. Connect the Nook to your computer, and navigate to the screensavers directory. Create a sub-directory with a name of your choosing, and copy your images to this directory. Disconnect the Nook. On the Nook itself, navigate to Settings/Screen/screensavers. Select the directory name you created for your images.
Use ImageMagick to convert a group of color landscape images (*.jpg) into a format suitable for screensavers (ss1.jpg, ss2.jpg, ...):
-> convert -rotate 90 -colorspace gray *.jpg ss%d.jpg