Cambiar magen de fondo del gdm y lockscreen

The default background is that zoomed in picture of a green leaf. It appears for newly created users, but more importantly, this is the image shown when the screen is locked. As of 25-Apr-2009, you can find this image here

/usr/share/pixmaps/backgrounds/gnome/background-default.jpg

To change it, simply copy your favorite image to this location (as root) and rename it.