vi /usr/share/gdm/greeter-config/20_debian
/desktop/gnome/background/picture_filename /usr/share/images/desktop-base/login-background.svg
/desktop/gnome/interface/gtk_theme Clearlooks
/desktop/gnome/background/picture_filename /usr/share/images/SUA_IMAGEM.jpg
dpkg-reconfigure gdm3
Encerre a sessão e veja se tudo deu certo!