Desktop backgrounds keep switching position

I've had a problem with my Mac Pro since I first bought it. I have a setup with six 1920x1080 screens (3 screens wide, 2 screens high). I've processed some nice (large) photos, so I could set them on these screens and still make it look like one big photo. I've calculated and cut out the bezels from the photos and it looked very nice. The problem though: my Mac can't seem to remember which one goes where. An example:

Looks nice, doesn't it? Yet, I regularly boot my Mac and see this:

The photos that are actually part of my background have been moved to a different screen and then there are two parts from older desktop background that have been reintroduced (bottom left and right).

I've tried removing every single folder from the background selection dialog window, and just adding the one I want (if that helped, I could live with it), but it doesn't seem to help. Every single OSX update (I've just updated tot 10.11.3) gives me a little hope that it's fixed, but it never is.

Do you guys have any idea how I can keep the backgrounds in place, or, stretch a single image between multiple monitors (preferably without third party software).

In case you wonder, the Mac does remember where the displays are relative to each other. I can move my cursor between screens without a problem.

I look forward to your response(s).

Mac Pro, OS X El Capitan (10.11.3)

Try to do these things.

1. Check that you have this option unchecked. I am sure you have it unchecked, but just in case :) It may be set to 'Change picture: When logging in'. So let's make sure that this is not the reason.

2. Then you may try to enter following command in terminal:

   rm ~/Library/Preferences/com.apple.Desktop.­plist

   After this, set your desktop pictures again, reboot and see if it helped.

3. This may be strange, but try to uncheck this option:

It's not actually resetting your backgrounds but creating a new desktop space on every boot. Check your spaces by pressing the mission control button (F3 on mac keyboard), it should show a bunch of desktops.

Anyway, let me know the result (via comment). Question is interesting.