Warning: I tested this mod on my Story HD Basic, but I can't guarantee it will work for you. Use at your own risk!
it's possible to replace the Story HD's screensaver images with custom ones. I've done this successfully on my Story HD Basic. It should work for other models (US Wifi etc.) as well, but I can't test this myself. Unfortunately the number of images can not be increased with this method (it's four images in total).
The attached firmware update file moves the four images from system storage to the USB-accessible internal storage. It creates a folder /sleep/ which contains the four images, which you can then simply replace with other files (PNG, 768x1024, Grayscale).
To use, rename the attached storyeb07-screensaver.hex to storyeb07.hex and copy it to the internal memory (like a regular firmware update).
This change can also be reverted by using the attached Undo file. It will also delete the /sleep/ folder from internal storage.
Update 04 Mar 2012:
Screensaver images are now displayed without the slide-power-button-to-turn-back-on message. It was unreadable on half of my images and in the way in the other half, and I assume people who install mods already know how to turn it back on. If you miss the text just add it to the image itself.