"source" locks at "loading" and no image-screensaver can be selected

First follow the instructions in this support article.

If an iPhoto or Aperture library is the source of your desktop pictures, it may need to be rebuilt, or you may need to update the application to the latest version.

Otherwise, continue as below.

Back up all data.

Launch the Console 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 Console in the icon grid.

Make sure the title of the Console window is All Messages. If it isn't, select All Messages from the SYSTEM LOG QUERIES menu on the left. If you don't see that menu, select

View ▹ Show Log List

from the menu bar.

Click the Clear Display icon in the toolbar. Then try the action that you're having trouble with again. Select any messages that appear in the Console window. Copy them to the Clipboard (command-C). Paste into a reply to this message (command-V).

When posting a log extract, be selective. In most cases, a few dozen lines are more than enough.

Please do not indiscriminately dump thousands of lines from the log into this discussion.

Important: Some private information, such as your name, may appear in the log. Anonymize before posting.

Back up all data.

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

~/Library/Preferences/com.apple.iApps.plist

Services ▹ Reveal

from the contextual menu.* A folder should open with an item selected. Quit Aperture and iPhoto if they're running. Move the selected item to the Desktop, leaving the window open. Test. If there's no change, quit again and put the item you moved back where it was, overwriting the one that may have been created in its place. Otherwise, delete the item you moved.

*If you don't see the contextual menu item, copy the selected text to the Clipboard (command-C). In the Finder, select

Go ▹ Go to Folder...

from the menu bar, paste into the box that opens (command-V). You won't see what you pasted because a line break is included. Press return.

I don't use Aperture so I can't test, but it sounds to me like you're triggering a bug. There are a couple of discussions of this issue with ideas that you might find helpful:

Aperture Library unavailable in Desktop Background

Desktop pictures

None of the suggestions in those threads helped?

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.

The purpose of this test is to determine whether the problem is localized to your user account. Enable guest logins* and log in as Guest. 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. Create a photo library by downloading some images from the Web or importing from the folder /Library/Desktop Pictures. Same problem?

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 in OS X 10.7 or later, 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.

Problems such as yours are sometimes caused by files that should belong to you but are locked or have wrong permissions. This procedure will check for such files. It makes no changes and therefore will not, in itself, solve your problem.

First, empty the Trash.

Triple-click the line below on this page to select it, then copy the selected text to the Clipboard (command-C):

find ~ $TMPDIR.. \( -flags +sappnd,schg,uappnd,uchg -o ! -user $UID -o ! -perm -600 -o -acl \) 2> /dev/null | wc -l

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 into the Terminal window (command-V). The command may take a noticeable amount of time to run. Wait for a new line ending in a dollar sign (“$”) to appear.

The output of this command, on a line directly below what you entered, will be a number such as "41." Please post it in a reply.

Back up all data. Don't continue unless you're sure you can restore from a backup, even if you're unable to log in.

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. To do that, unlock the preference pane using the credentials of an administrator, check the box marked Allow user to administer this computer, then reboot. You can demote the problem account back to standard status when this step has been completed.

Enter the following command in the Terminal window in the same way as before (triple-click, copy, and paste):

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

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 to be careful. 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 (optional)

Take this step only if you have trouble with Step 1 or if it doesn't solve the problem.

Boot into Recovery. 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:

res

Press the tab key. The partial command you typed will automatically be completed to this:

resetpassword

Press return. A Reset Password window will open. You’re not 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.