If you run 'gconf-editor' you get visual interface to change the keys & values.
For background you should edit value 'picture_filename' of key 'desktop/gnome/background'
If you want to do it in terminal then there is a command 'gconftool-2'
gconftool-2 -t str -s /desktop/gnome/background/picture_filename Wallpaper_File_with_full_path
Actually this is fun because I telnet to my friend's PC & use
gconftool-2 -g /desktop/gnome/background/picture_filename
to find out his current wallpaper then I can change it (with his consent) & he will notice it.
Like , when I want to show him the tricks of 'imagemagick' , I upload my desired wallpaper on his PC.
I am using ubuntu 8.04.
Pleased to help.