Update your wallpapers regularly (please don't be frequently) by polling wallpaperswide.com.

wallpapers.sh

#!/bin/bash
# Update your wallpapager regularly.
# Preferrably installing it in your crontab:
#
# (crontab -l | sed '1,3d'; echo "# Update wallpager every half an hour."; echo "0,30 * * * * /path/to/wallpaper.sh") | crontab -
#
# by Jerry Fleming <jerryfleming2006@gmail.com>
# on 2013-03-25.
# No rights reserved. Use at your own risk.

# Your probably don't need this proxy setting.
[ -e /etc/proxy ] && source /etc/proxy

# Your preferred place to store the pictures. Please mkdir and chown as appropriate.
FILE=/var/tmp/wallpaper
# Time to sleep before every web request. Shorter values may get your IP to be blocked.
DELAY=1.0

# Start over again every day, in case the ranking is changed.
now=$(date +%s)
mtime=$(date +%s -r $FILE.seq 2>/dev/null)
age=$(($now - ${mtime:-0}))
[ $age -gt 8640000 ] && {
    rm $FILE.name
    touch $FILE.seq
}

# See if a user is logged in over a desktop session.
GUI_SESSIONS=$( who | grep "$USER.*(:" | wc -l )
[ $GUI_SESSIONS -eq 0 ] && exit 1

{ read oname page cnt < $FILE.name ; } 2>/dev/null
page=${page:=1}
cnt=${cnt:=1}
same_page=1
while true
do
    item=0
    curl -s http://wallpaperswide.com/nature-desktop-wallpapers/page/$page \
    | sed -n 's#.*<a href="/\([^-]\+\)-wallpapers.html.*#\1#p' \
    | uniq \
    | while read name
    do
        item=$(($item+1))
        # If this is already another page, item count doesn't matter.
        [ $same_page -eq 1 ] && [ $item -lt $cnt ] && continue
        # Skip the old item itself.
        [ x"$oname" == x"$name" ] && continue
        [ -e ~/wallpapers/$name.jpg ] && continue
        sleep $DELAY
        curl -s http://wallpaperswide.com/download/$name-wallpaper-1920x1080.jpg \
            -o $FILE.jpg.new -D $FILE.header \
            -e http://wallpaperswide.com/$name-wallpapers.html
        # Either it doesn't exist or there is an error.
        grep Location $FILE.header > /dev/null && continue
        # Ensure that the picture comes at once, to avoid flickering/partial loading.
        mv $FILE.jpg.new $FILE.jpg
        cp $FILE.jpg ~/wallpapers/$name.jpg
        echo $name $page $item > $FILE.name
        break
    done
    [ -e $FILE.header ] && break
    page=$(($page+1))
    same_page=0
    sleep $DELAY
done
rm -f $FILE.header

# This only need to be done once, but doesn't matter if done repeatedly.
gconftool-2 --type string --set /desktop/gnome/background/picture_filename $FILE.jpg

exit 0