Desktop backgrounds keep switching position
Try to do these things.


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.





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.
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.