I found solution!!!
to change Mageia 5 GNOME 3 gdm login screen background - wallpaper you must run in terminal —
su
nautilus /usr/share/mga/backgrounds/
then you will see something like this
in /usr/share/mga/backgrounds/ folder there is a symlink with name "default.jpg", that point in my case to /usr/share/mga/backgrounds/Mageia-Default-1600x1200.jpg,
'Mageia-Default-1600x1200.jpg' — is a gnome 3 login screen background, to change it — copy your custom background to /usr/share/mga/backgrounds/,
rename original 'Mageia-Default-1600x1200.jpg' to 'Mageia-Default-1600x1200.jpg.bak', rename your custom image to 'Mageia-Default-1600x1200.jpg'...
relogin and enjoy your custom gdm background!!! sorry for my english)
Thanks to all! Mageia GNOME is the BEST! )