I used the following entry for the Fedora 15 based remix (in my kickstart file) which I have used for the Fedora 14 based remix.
But there is no change on default wallpaper.
gconftool-2 --direct --config-source=xml:readwrite:/etc/gconf/gconf.xml.defaults -s -t string /desktop/gnome/background/picture_filename /usr/share/backgrounds/gnome/feel-the-freedom.jpg >/dev/null
And also I noticed images those added by myself are resides under /usr/share/backgrounds/gnome/ directory but I could not find them when I open the window to change the background.
Appreciate your help.