[neil_swann80]

If you use KOReader, Gruntbug's solution above is automatic and convenient. If you don't use KOReader, or just prefer a more manual approach then continue below...

I've written a simple binary that should make setting the sleep image as a book cover a tad easier.
It will also set the Boot Logo and Power-off Logo at the same time.
After the initial install manually change Settings>Personalize>Logos for both the Boot Logo and Power-off Logo to: SetSleep

Once installed you will be able to long-press on a book on your home or library view and select:
Open with > Set as sleep
This will create the sleep cover image and reboot the device.

Install instructions:

Spoiler:

Upload the attached file to the root directory of your device storage (/mnt/ext1) and rename it without the .zip extension, e.g. SleepScreen.pbi

After unplugging the USB (or a reboot) open the Library, select "Formats" from the drop-down menu, then select "PBI". Click on SleepScreen and choose install.

Manually change Settings>Personalize>Logos for both the Boot Logo and Power-off Logo to: SetSleep

With that complete, you will be able to long-press on a book on your home or library view and select:
Open with > Set as sleep
This will create the sleep cover image and reboot the device.

EDIT: Added functionality

Spoiler:

Added a dialog window with 3 options...

CREATE - Creates the sleep image from book cover and restarts.

RESTORE - Restores the sleep image to the default & restarts:

/mnt/ext1/system/resources/Line/old/taskmgr_lock_background.bmp
Attempts to first restore file above, if this file doesn't exist it restores the Pocketbook default.

EXIT - Exits without a restart.