set-gdm-wallpaper.sh /path/to/image.png
# (use --resize N option for fixing multiple monitor issues)
# Script requires gresource binary (glib2 or glib2-devel library):
# Fedora: 
dnf install glib2-devel

Fedora Silverblue have immutable file system. GDM wallpaper can be changed only by install/reinstall rpm.
# rpm-ostree install set-gdm-wallpaper-1-2.noarch.rpm
This will change GDM wallpaper to wallpaper-gnome.png inside .rpm file.
If you want change this image, you need to Build new .rpm file.

$ dnf copr enable zirix/gdm-wallpaper
$ dnf install gdm-wallpaper
$ set-gdm-wallpaper /path/to/image.png

# dnf install set-gdm-wallpaper-1-2.noarch.rpm

# rpm-ostree install set-gdm-wallpaper-1-2.noarch.rpm

# set-gdm-wallpaper.sh --uninstall
OR
# cp /usr/share/gnome-shell/gnome-shell-theme.gresource.backup /usr/share/gnome-shell/gnome-shell-theme.gresource
OR delete RPM:
# dnf remove set-gdm-wallpaper
OR
# rpm-ostree uninstall set-gdm-wallpaper

$ cd /path/to/source
$ fedpkg --release f29 local
$ fedpkg --release f29 lint
Requires: fedora-packager and fedora-review
# dnf install fedora-packager fedora-review
OR
# rpm-ostree install fedora-packager fedora-review
See: https://docs.fedoraproject.org/en-US/quick-docs/creating-rpm-packages/index.html