Mavericks does not respect MCX applied to com.apple.desktop.plist. This script - which can be called via a LaunchAgent for every user that logs in - will read the plist file and extract the desktop image set by MCX. Normally this key is found in Background > default > ImageFilePath but sometimes (perhaps related to not being managed? or Mission …

SetStudentDesktop.py

#!/usr/bin/env python

# 1.1	2014-03-04 BP rewrote to use complete.plist MCX since the pref file method was unreliable
# 1.0   2014-02-23 BP first release for use with 10.9.x base images

# based on gist published by Greg Neagle: https://gist.github.com/gregneagle/6957826
# only needed (and tested) on 10.9, but may work on older OS versions

from AppKit import NSWorkspace, NSScreen
from Foundation import NSURL
import getpass
import CoreFoundation

#import subprocess, sys, plistlib, json 
import plistlib

def SetDesktop(imagepath):
	file_url = NSURL.fileURLWithPath_(imagepath)
	options = {}	#  empty options dict is ok because the defaults are ok
	ws = NSWorkspace.sharedWorkspace()

	for screen in NSScreen.screens():
		(result, error) = ws.setDesktopImageURL_forScreen_options_error_(file_url, screen, options, None)
	return;


def GetDesktopImagePathFromCompleteMCX():
	path=""
	mcxpath="/Library/Managed Preferences/" + username + "/complete.plist"
	pl = plistlib.readPlist(mcxpath) 
	if pl:
		key_desktop = pl.get('com.apple.desktop')
		if key_desktop:
			key_background = key_desktop.get('Background')
			if key_background:
				key_value = key_background.get('value')
				if key_value:
					key_default = key_value.get('default')
					if key_default:
						path = key_default.get('ImageFilePath')
						if path:
							print "success! managed desktop background key found: " + path

	# can't find the correct key, no way to recover so use a hard coded fallback path
	if path == "":
		print "### ERROR: unable to find managed desktop background key - defaulting to Ladybug"
		path="/Library/Desktop Pictures/Nature/Ladybug.jpg"	

	return path
	

# student accounts always start with an 's' so we can test for that
username=getpass.getuser()

if username[0] == 's':
	imagePath=GetDesktopImagePathFromCompleteMCX()
	print "[" + username + "] starts with an s, setting desktop picture to %s" % imagePath
	SetDesktop(imagePath)
else:
	print "[" + username + "] does not start with an s, desktop unchanged"