Changing Desktop Picture

First, open Disk Utility, choose Macintosh HD in the sidebar and repair permissions.

If it doesn't work, open Finder, select Go menu > Go to Folder, type ~/Library/Preferences, delete "com.apple.desktop.plist" and "com.apple.systempreferences.plist", and restart the Mac. Finally, change its wallpaper and it should work correctly

If you've enabled automatic login, disable it and test. Make sure you know your login password.

Did you delete those files when you were logged in your second user? If not, do it

It may be a drastic action, but open Finder, select Go menu > Go to Folder, and go to these directories:

~/Library/Caches

~/Library/Preferences

Empty these folders and restart

When you deleted the file named "com.apple.desktop.plist" in the problem account, was a new one created in its place?

Back up all data.

Triple-click anywhere in the line below to select it:

ls -@Odeln ~/L*/Preferences

Copy the selected text to the Clipboard (command-C).

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 icon grid.

Paste (command-V) into the Terminal window.

Post any lines of output that appear below what you entered — the text, please, not a screenshot.

Back up all data.

This procedure will unlock all your user files (not system files) and reset their ownership and access-control lists to the default. If you've set special values for those attributes on any of your files, they will be reverted. In that case, either stop here, or be prepared to recreate the settings if necessary. Do so only after verifying that those settings didn't cause the problem. If none of this is meaningful to you, you don't need to worry about it.

Step 1

If you have more than one user account, and the one in question is not an administrator account, then temporarily promote it to administrator status in the Users & Groups preference pane. You can demote it back to standard status when this step has been completed.

Triple-click the following line to select it. Copy the selected text to the Clipboard (command-C):

{ sudo chflags -R nouchg,nouappnd ~ $TMPDIR.. ; sudo chown -Rh $UID:20 ~ $_ ; chmod -R -N ~ $_ ; } 2> /dev/null

Paste into the Terminal window (command-V). 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. If you don’t have a login password, you’ll need to set one before you can run the command. If you see a message that your username "is not in the sudoers file," then you're not logged in as an administrator.

The command will take a noticeable amount of time to run. Wait for a new line ending in a dollar sign (“$”) to appear, then quit Terminal.

Step 2

Boot into Recovery by holding down the key combination command-R at startup. Release the keys when you see a gray screen with a spinning dial.

When the OS X Utilities screen appears, select

Utilities ▹ Terminal

from the menu bar. A Terminal window will open.

In the Terminal window, type this:

resetpassword

That's one word, all lower case, with no spaces. Then press return. A Reset Password window will open. You’renot going to reset a password.

Select your boot volume ("Macintosh HD," unless you gave it a different name) if not already selected.

Select your username from the menu labeled Select the user account if not already selected.

Under Reset Home Directory Permissions and ACLs, click the Reset button.

Select

 ▹ Restart

from the menu bar.

Step 1 is to be carried out in the problem account. In step 2, you're not logged in at all. As I wrote above, permissions and access-control lists will be reset. If you don't know what that means, you haven't (intentionally) changed them.