Use the recent "Astronomy Picture of the Day" by NASA as your desktop wallpaper (Python script for Mac OS X, should be easy do adapt for Linux or Windows)

change_wallpaper

#!/usr/bin/python
# Downloads the current "Astronomy Picture of the Day" from nasa.gov and sets as wallpaper
# 
# Installation steps:
# - store this file somewhere and take note of the path
# - change WALLPAPER_DIR to some folder in your home directory
#   and create the folder
# - make it executable:
#   chmod a+x /path/to/change_wallpaper
# - follow instructions given in the .plist file to make it run daily

FEED_URL = "http://apod.nasa.gov/apod/"
WALLPAPER_DIR = "/Users/mw/Pictures/Wallpapers/APOD"

import subprocess, re, datetime
from urllib import urlopen, urlretrieve
import os.path, os

def main():
    xml = urlopen(FEED_URL).read()

    m = re.search(r"a href=\"(image/.*?)\"", xml)
    
    #m = re.search(r"img src=\"(.*?)\"", xml)
    first_img = FEED_URL + m.group(1)
    name = re.sub(r"^.*/", "", first_img)
    filename = "%s/%s-%s" % (WALLPAPER_DIR, str(datetime.date.today()), name)

    print "Image URL: ", first_img
    print "Filename: ", name
    print "Target file: ", filename
    
    if os.path.isfile(filename):
        print "File already exists."
    else:
        print "Downloading..."
        urlretrieve(first_img, filename)

    set_desktop_background(filename)

SCRIPT = """/usr/bin/osascript -s o<<END
tell application "Finder"
set desktop picture to POSIX file "%s"
end tell
END"""

def set_desktop_background(filename):
    print "Changing wallpaper..."
    #subprocess.Popen(SCRIPT%filename, shell=True)
    subprocess.check_call(SCRIPT%filename, shell=True)

main()

io.weller.max.changewallpaper.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC -//Apple Computer//DTD PLIST 1.0//EN
http://www.apple.com/DTDs/PropertyList-1.0.dtd >
<plist version="1.0">
<dict>
  
  <key>Label</key>
  <string>io.weller.max.changewallpaper</string>
  
  <!--
    store this file in ~/Library/LaunchAgents/
    edit the path to change_wallpaper below
  -->
  
  <key>Program</key>
  <string>/path/to/change_wallpaper</string>
  
  <key>RunAtLoad</key>
  <true/>
  
  <key>StartCalendarInterval</key>
  <dict>  
      <key>Hour</key>
      <integer>3</integer>
      <key>Minute</key>
      <integer>15</integer>
  </dict>
</dict>
</plist>