W. Change/Restore desktop background - Change/Restore your desktop backgrounds (Thanks to kuzmichov and cckwes)...
Explanation (2 images):
* If you activated landscape mode, and you want to show the two images, one for portrait (on all 3 screens), and one for landscape (on all 3 screens), you can activate the option 1, make 2 pictures (one for landscape and one for the portrait.)

1. Create 2 pictures named:
1) “Pdeskbg1” with an 480x854 px dimension and with .png extension (Pdeskbg1.png) for portrait.
2) “Ldeskbg1” with an 854x480 px dimension and with .png extension (Ldeskbg1.png) for landscape.
2. Put this images from your PC to: /home/user/MyDocs/Pictures
3. Open N9 QTweak app (password of devel-su, default: rootme), choose w [hit Enter], then choose 1 [hit Enter].
4. Your setting will take effect after reboot. You can choose to restart immediately or not.

* If you use only portrait mode, it is enough to make one picture (Pdeskbg1.png) and on all three screens, you will have exact same picture...