Q: Mountain Lion desktop wallpaper won't change!

Quit System Preferences if it's running. In the Finder, hold down the option key and select Go ▹ Library from the menu bar. From the Library folder, delete the following item, if it exists:

Caches/com.apple.systempreferences

and move the following item to the Desktop:

Preferences/com.apple.desktop.plist

Launch System Preferences and test. If you still have the issue, put the item on the Desktop back where it came from and post again. Otherwise, delete the item.

Please read this whole message before doing anything.

This procedure is a test, not a solution. Don’t be disappointed when you find that nothing has changed after you complete it.

Step 1

The purpose of this step is to determine whether the problem is localized to your user account.

Enable guest logins* and log in as Guest. For instructions, launch the System Preferences application, select Help from the menu bar, and enter “Set up guest users” (without the quotes) in the search box. Don't use the Safari-only “Guest User” login created by “Find My Mac.”

While logged in as Guest, you won’t have access to any of your personal files or settings. Applications will behave as if you were running them for the first time. Don’t be alarmed by this; it’s normal. If you need any passwords or other personal data in order to complete the test, memorize, print, or write them down before you begin.

Test while logged in as Guest. Same problem(s)?

After testing, log out of the guest account and, in your own account, disable it if you wish. Any files you created in the guest account will be deleted automatically when you log out of it.

*Note: If you’ve activated “Find My Mac” or FileVault, then you can’t enable the Guest account. The “Guest User” login created by “Find My Mac” is not the same. Create a new account in which to test, and delete it, including its home folder, after testing.

Step 2

The purpose of this step is to determine whether the problem is caused by third-party system modifications that load automatically at startup or login.

Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards. Boot in safe mode* and log in to the account with the problem. The instructions provided by Apple are as follows:

1. Be sure your Mac is shut down.
2. Press the power button.
3. Immediately after you hear the startup tone, hold the Shift key. The Shift key should be held as soon as possible after the startup tone, but not before the tone.
4. Release the Shift key when you see the gray Apple icon and the progress indicator (looks like a spinning gear).

Safe mode is much slower to boot and run than normal, and some things won’t work at all, including wireless networking on certain Macs.

The login screen appears even if you usually log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.

*Note: If FileVault is enabled, or if a firmware password is set, you can’t boot in safe mode.

Test while in safe mode. Same problem(s)?

After testing, reboot as usual (i.e., not in safe mode) and verify that you still have the problem. Post the results of steps 1 and 2.

Were you able to change the desktop picture while you were in safe mode?

Please read this whole message before doing anything.

This procedure is a diagnostic test. It won’t solve your problem. Don’t be disappointed when you find that nothing has changed after you complete it.

Third-party system modifications are a common cause of usability problems. By a “system modification,” I mean software that affects the operation of other software — potentially for the worse. The following procedure will help identify which such modifications you've installed. Don’t be alarmed by the complexity of these instructions — they’re easy to carry out and won’t change anything on your Mac.

These steps are to be taken while booted in “normal” mode, not in safe mode. If you’re now running in safe mode, reboot as usual before continuing.

Below are instructions to enter some UNIX shell commands. The commands are harmless, but they must be entered exactly as given in order to work. If you have doubts about the safety of the procedure suggested here, search this site for other discussions in which it’s been followed without any report of ill effects.

Some of the commands will line-wrap or scroll in your browser, but each one is really just a single line, all of which must be selected. You can accomplish this easily by triple-clicking anywhere in the line. The whole line will highlight, and you can then either copy or drag it. The headings “Step 1” and so on are not part of the commands.

Note: If you have more than one user account, Step 2 must be taken as an administrator. Ordinarily that would be the user created automatically when you booted the system for the first time. The other steps should be taken as the user who has the problem, if different. Most personal Macs have only one user, and in that case this paragraph doesn’t apply.

Launch the Terminal application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad. Click Utilities, then Terminal in the page that opens.

When you launch Terminal, a text window will open with a line already in it, ending either in a dollar sign (“$”) or a percent sign (“%”). If you get the percent sign, enter “sh” and press return. You should then get a new line ending in a dollar sign.

Step 1

Copy or drag — do not type — the line below into the Terminal window, then press return:

kextstat -kl | awk '!/com\.apple/{printf "%s %s\n", $6, $7}'

Post the lines of output (if any) that appear below what you just entered (the text, please, not a screenshot.) You can omit the final line ending in “$”.

Step 2

Repeat with this line:

sudo launchctl list | sed 1d | awk '!/0x|com\.(apple|openssh|vix)|edu\.mit|org\.(amavis|apache|cups|isc|ntp|postfix|x)/{print $3}'

This time, you'll be prompted for your login password, which won't be displayed when you type it. You may get a one-time warning not to screw up. You don't need to post the warning.

Note: If you don’t have a login password, you’ll need to set one before taking this step. If that’s not possible, skip to the next step.

Step 3

launchctl list | sed 1d | awk '!/0x|com\.apple|edu\.mit|org\.(x|openbsd)/{print $3}'

Step 4

ls -1A /e*/mach* {,/}L*/{Ad,Compon,Ex,Fram,In,Keyb,La,Mail/Bu,P*P,Priv,Qu,Scripti,Servi,Spo,Sta}* L*/Fonts 2> /dev/null

Important: If you formerly synchronized with a MobileMe account, your me.com email address may appear in the output of the above command. If so, anonymize it before posting.

Step 5

osascript -e 'tell application "System Events" to get name of every login item' 2> /dev/null

Remember, steps 1-5 are all drag-and-drop or copy-and-paste, whichever you prefer — no typing, except your password. Also remember to post the output.

You can then quit Terminal.

Please read this whole message carefully, especially the warnings, before doing anything.

1. The changes to your configuration suggested here should be considered provisional; they may not solve your problem, or they may remove functionality that you find useful. If a third-party system modification that you want to keep is causing the problem, seek help from its developer.

2. WARNING: Back up all data now if you haven’t already done so. Before proceeding, you must be sure you can restore your system to its present state, even if it becomes unbootable. If you’re not sure you can do that, STOP — DON’T CHANGE ANYTHING. If you’re dissatisfied with the results of the procedure suggested below, restore from your backup. I will not be responsible for the consequences, and I will not be able to help, if you ignore this warning.

3. You should either remove or update the following system modification(s), if an update is available from the developer:

N/A

and definitely remove at least the following:

† VirusScan

4. Whatever you remove must be removed completely, and (unless otherwise specified in this message) the only way to do that is to use the uninstallation tool, if any, provided by the third-party developers, or to follow their instructions. In some cases it may be necessary to re-download or even reinstall the software in order to get rid of it. I can't be more specific, because I don't install such things myself. Please do your own research.

Here are some general guidelines to get you started. Suppose you want to remove something called “BrickYourMac.” First, consult the product's Help menu, if there is one, for instructions. Finding none there, look on the developer's website, say www.brickyourmac.com. (That may not be the actual name of the site; if necessary, search the web for the product name.) If you don’t find anything on the website or in your search, email the developer. While you're waiting for a response, download BrickYourMac.dmg and open it. There may be an application in there such as “Uninstall BrickYourMac.” If not, open “BrickYourMac.pkg” and look for an Uninstall button.

Again, please don't ask me to do this research for you. You can do it better than I can, because I haven't installed the product and I may not even know what it is.

If you can’t remove software in any other way, you’ll have to erase your boot volume and perform a clean reinstallation of OS X. Never install any third-party software unless you're sure you know how to uninstall it; otherwise you may create problems that are very hard to solve.

WARNING: Trying to remove complex system modifications by hunting for files by name often will not work and may make the problem worse. The same goes for "utilities" that purport to remove software.

5. I recommend that you never reinstall the modifications marked with a dagger (†) above, if any. If your problem is resolved after uninstalling all the above modifications and rebooting, but you still want to use some of those not marked with a dagger, you can experiment with putting them back, one at a time, testing carefully after each step. Keep in mind that system modifications may be incompatible with each other or with future OS X updates, so it may not be clear which one is at fault.

6. If you still have problems after making the suggested changes and rebooting, post again. Remember: if you don’t like the results of this procedure, you can undo it by restoring from the last backup you made before you started.

I found out that I have the same issue. Deleting Caches/com.apple.systempreferences and Preferences/com.apple.desktop.plist did not helped. Finally I found out that is somehow related to login screen. If I enable login screen I can change the wallpaper picture without reverting to an old one after reboot. Unfortunately keeping the login screen is leading to another issue that seems to be still unsolved by Apple which is the "black screen after login" issue.

I've been having this same problem ever since I got my Retina Display! I'm so glad I'm not the only one. I was getting ready to make a Genius Bar appointment. I'm so sick of seeing this **** elephant in my background. Anyways, I want to try the login screen but I can't even figure out how to enable it...

Updating to 10.8.2 did not solved the problem?

Anyway, to enable login screen you'll have to go 'System Preferences/Security&Privacy/Disable Automatic Login' unchecked.