/usr/share/gdm/autostart/LoginWindow/mythbuntu-gdm-background-picture.desktop is in mythbuntu-gdm-theme 0.6-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | [Desktop Entry]
Encoding=UTF-8
Name=GDM Background picture
Comment=Set GDM background picture
Icon=image-viewer
Exec=feh --no-menus --bg-scale /usr/share/images/xsplash/bg_1920x1200.jpg
Terminal=false
Type=Application
Categories=
OnlyShowIn=GNOME;XFCE;
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