Update: corrected off-by-one error thank to jwkrahn.

#!/usr/bin/perl

# Gnome random wallpaper picker.
# 
# Copyright (C) 2010 Alessandro Ghedini <al3xbio@gmail.com>
# --------------------------------------------------------------
# "THE BEER-WARE LICENSE" (Revision 42):
# Alessandro Ghedini wrote this file. As long as you retain this 
# notice you can do whatever you want with this stuff. If we 
# meet some day, and you think this stuff is worth it, you can 
# buy me a beer in return.
# --------------------------------------------------------------

use Gnome2::GConf;
use XML::Simple;

use strict;
use warnings;

my $client  = Gnome2::GConf::Client -> get_default;
my $bg_file = $ENV{'HOME'}.'/.gnome2/backgrounds.xml';
my $key     = '/desktop/gnome/background/picture_filename';

open BG, $bg_file or die $!;
my $xml_data = join '', <BG>;
close BG;

my $xml = XMLin($xml_data);

my @wallpapers;

while (my ($key, $value) = each(%{$xml -> {'wallpaper'}})){

    push @wallpapers, $key if $value -> {'deleted'} eq 'false';

}

my $name     = $wallpapers[rand(@wallpapers)];
my $selected = $xml -> {'wallpaper'} -> {$name};

print "Selected Wallpaper: $name\n";

$client -> set($key, {type => 'string',
              value => $selected -> {'filename'}});

__END__

=head1 NAME

gnome-random-wallpaper.pl - Gnome random wallpaper picker.

=head1 USAGE

gnome-random-wallpaper.pl

=head1 DESCRIPTION

This script simply reads the ~/.gnome2/backgrounds.xml file, where are
stored the default wallpapers for Gnome, randomly selects one of them,
and sets it using the GConf interface.

=cut
[download]

The usage is very simple, just run it without any arguments.