Step 1: Edit this file /etc/gdm3/greeter.gconf-defaults
sudo gedit /etc/gdm3/greeter.gconf-defaults
Look for #Use specific background
Uncomment or add your own line (The absolute path to myimage.jpg is an example on where my image is)
/desktop/gnome/background/picture_filename /home/user/wallpaper/myimage.jpg
Step2 : sudo dpkg-reconfigure gdm3
This reconfigure the gdm3 config, without doing this your gdm background image will never change.
Step 3: logout, you should see the background image you specified.
cyruslab 
Thanks for this info I googled this and found your blog – brilliant, just the info I needed