Does your account have necessary permissions to make changes? You may want to check it by typing in terminal group and it will show what groups you belong to.

Next thing, you can actually change the user background the hard way. There is a profile folder for each user stored in /var/lib/AccountsService/users. For instance, /var/lib/AccountsService/users/sergiy is my account profile. What you can do is to edit that file with the text editor of your choice. I prefer nano. So what I type is this:

sudo nano /var/lib/AccountsService/users/sergiy

It will prompt for your password (considering that you're in sudoer group, of course) and open the file. Locate line starting with [User] and line saying Background . Paste the path to your desired wallpaper after = sign. For instance, here's mine:

After you're done editing , reboot.

And one more thing, I've discovered that file org.freedestop.Accounts.service prevents changes to greeter backgrounds, and most likely to user backgrounds. So try editing this file with a text editor to comment out Exec line. Again, for me this is

sudo nano /usr/share/dbus-1/system-services/org.freedesktop.Accounts.service

Change Exec=/usr/lib/accountsservice/accounts-daemon to # Exec=/usr/lib/accountsservice/accounts-daemon

Try this first, and let me know if this helps. I've discovered that after commenting out that line, I was able to change lightdm-gtk greeter background, while previously I could not - it would just override whatever settings I placed into lightdm-gtk-greeter.conf, and show my desktop background instead of the image i wanted.

Note: keep in mind that the first solution I posted, will only work if that Exec line is active, not commented out. Second solution, disables Accounts service

Let me know if this helps. Cheers