How can I programmatically change the background in Mac OS X?

If you are doing this for the current user, you can run, from a shell:

defaults write com.apple.desktop Background '{default = {ImageFilePath = "/Library/Desktop Pictures/Black & White/Lightning.jpg"; };}'

Or, as root, for another user:

/usr/bin/defaults write /Users/joeuser/Library/Preferences/com.apple.desktop Background '{default = {ImageFilePath = "/Library/Desktop Pictures/Black & White/Lightning.jpg"; };}'
chown joeuser /Users/joeuser/Library/Preferences/com.apple.desktop.plist

You will of course want to replace the image filename and user name.

The new setting will take effect when the Dock starts up -- either at login, or, when you

killall Dock

[Based on a posting elsewhere, and based on information from Matt Miller's answer.]

From python, if you have appscript installed (sudo easy_install appscript), you can simply do

from appscript import app, mactypes
app('Finder').desktop_picture.set(mactypes.File('/your/filename.jpg'))

Otherwise, this applescript will change the desktop background

tell application "Finder"
    set desktop picture to POSIX file "/your/filename.jpg"
end tell

You can run it from the command line using osascript, or from Python using something like

import subprocess

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

def set_desktop_background(filename):
    subprocess.Popen(SCRIPT%filename, shell=True)