/home/"username"/.gconf/desktop/gnome/background/%gconf.xml
Example Fedora 10 : %gconf.xml , text shown in a texteditor :
Code:
#22224b4b6868 #14142c2c3d3d /usr/share/backgrounds/solar/solar.xml vertical-gradient stretched
May be not equal to this on all versions of gnome.
.....