无独有偶

ownloaded this: https://github.com/elementary/greeter/a ... master.zip

from https://github.com/elementary/greeter/

Extracted the zip to /tmp

I opened terminal, sudo -i, typed root password. Then run gimp as root (just type gimp in terminal), saved jpg file as png in greeter-master/data folder (replaced previous texture.png file).

Edited the SystemBackground.vala file located in /tmp/greeter-master/compositor/

sudo vi /tmp/greeter-master/compositor/SystemBackground.vala

Changed path from:
var texture_file = GLib.File.new_for_uri ("resource:///io/elementary/desktop/gala/texture.png");

to:
var texture_file = GLib.File.new_for_uri ("resource:///io/elementary/greeter/texture.png");

Installed all dependencies listed here:

https://github.com/elementary/greeter/b ... /README.md

gnome-settings-daemon >= 3.27
how to check version in terminal:
sudo gnome-control-center --version

Very important:
##############################
Installed dependencies:
sudo apt install -y libaccountsservice-dev libgdk-pixbuf2.0-dev libgranite-dev libgtk-3-dev liblightdm-gobject-1-dev libwingpanel-2.0-dev libx11-dev meson valac libmutter-2-0 libmutter-2-dev
##############################

cd /tmp/greeter-master
meson build --prefix=/usr
cd build
ninja
ninja install

It is working. Finally! Hell yeah!