How to change login background image

I have installed High Sierra on two different macs and I have the same strange situation.

This is the problem:

The login image background does not change if I change desktop background image.

When Mac starts up it shows this login image background.

Then I change the desktop background for example with this one

If mac stops it shows the correct desktop background when started

Also if I log out it shows the correct background image

But If I Restart or Switch off the Mac when it boots again still appears the same old image

I have tried all the tricks suggested:

1) the com.apple.desktop.admin.png in Machintosh HD>Library>Caches is correct. It shows the new desktop blurred image

I tried to delete it, reboot, but nothing

2) I have removed rm ~/Library/Preferences/com.apple.dock.plist.

Nothing

3) I have removed

cd ~/Library/Preferences

rm com.apple.desktop.plist

But nothing

4) I have tried to reset NVRAM but nothing

No way.

It seems that this background image is stored somewhere else and does not update with the new one.

I would like to have the same desktop image event in the start up login page.

Any idea?

This problems happens also in another macbook air that I have that I have just installed High Sierra.