Algum tempo atrás eu encontrei o seguinte script (não me lembro exatamente onde neste momento, mas quando eu vou encontrar, vou adicionar a fonte também) qual deles eu mudei um pouco e que está funcionando muito bem para o que você perguntou se está definido como um cron job (veja aqui como fazer isso):
#!/bin/bash
# export DBUS_SESSION_BUS_ADDRESS environment variable useful when the script is set as a cron job
PID=$(pgrep gnome-session)
export DBUS_SESSION_BUS_ADDRESS=$(grep -z DBUS_SESSION_BUS_ADDRESS /proc/$PID/environ|cut -d= -f2-)
# $bing is needed to form the fully qualified URL for
# the Bing pic of the day
bing="www.bing.com"
# $xmlURL is needed to get the xml data from which
# the relative URL for the Bing pic of the day is extracted
#
# The mkt parameter determines which Bing market you would like to
# obtain your images from.
# Valid values are: en-US, zh-CN, ja-JP, en-AU, en-UK, de-DE, en-NZ, en-CA.
#
# The idx parameter determines where to start from. 0 is the current day,
# 1 the previous day, etc.
xmlURL="http://www.bing.com/HPImageArchive.aspx?format=xml&idx=1&n=1&mkt=en-US"
# $saveDir is used to set the location where Bing pics of the day
# are stored. $HOME holds the path of the current user's home directory
saveDir="$HOME/Pictures/BingDesktopImages/"
# Create saveDir if it does not already exist
mkdir -p $saveDir
# Set picture options
# Valid options are: none,wallpaper,centered,scaled,stretched,zoom,spanned
picOpts="zoom"
# The desired Bing picture resolution to download
# Valid options: "_1024x768" "_1280x720" "_1366x768" "_1920x1200"
desiredPicRes="_1366x768"
# The file extension for the Bing pic
picExt=".jpg"
# Extract the relative URL of the Bing pic of the day from
# the XML data retrieved from xmlURL, form the fully qualified
# URL for the pic of the day, and store it in $picURL
# Form the URL for the desired pic resolution
desiredPicURL=$bing$(echo $(curl -s $xmlURL) | grep -oP "<urlBase>(.*)</urlBase>" | cut -d ">" -f 2 | cut -d "<" -f 1)$desiredPicRes$picExt
# Form the URL for the default pic resolution
defaultPicURL=$bing$(echo $(curl -s $xmlURL) | grep -oP "<url>(.*)</url>" | cut -d ">" -f 2 | cut -d "<" -f 1)
# $picName contains the filename of the Bing pic of the day
# Attempt to download the desired image resolution. If it doesn't
# exist then download the default image resolution
if wget --quiet --spider "$desiredPicURL"
then
# Set picName to the desired picName
picName=${desiredPicURL##*/}
# Download the Bing pic of the day at desired resolution
curl -s -o $saveDir$picName $desiredPicURL
else
# Set picName to the default picName
picName=${defaultPicURL##*/}
# Download the Bing pic of the day at default resolution
curl -s -o $saveDir$picName $defaultPicURL
fi
# Set the GNOME3 wallpaper
gsettings set org.gnome.desktop.background picture-uri "file://$saveDir$picName"
# Set the GNOME 3 wallpaper picture options
gsettings set org.gnome.desktop.background picture-options $picOpts
# Remove pictures older than 30 days
#find $saveDir -atime 30 -delete
# Exit the script
exit
