I recently replicated a problem experienced by other Mac users who are running OS X 10.5 and use Aperture to manage and edit their photo library. An entry on the Apple Discussion Board describes the essence of the problem:
“I’m seeing a bug in Systems Preferences desktop background selector. I am unable to add a folder to the sidebar because an Aperture Projects folder appears there instead. I can add the folder I want but then I have two identical Aperture folders and my selected folder is not visible.
If I then attempt to add the folder again, I get an error message telling me the folder already exists, when it in fact does not. I deleted the Desktop background preferences and it still does not work. I deleted Aperture and rebooted and it still does not work. Is this a known bug? How do I fix it? Is there any way to delete the Aperture Projects folder from the Prefpanel? Aperture has also wrecked my Screensaver. All I get now is an error message that no pictures are in the selected folder.”
A bug report about this same issue has also been posted on MacRumors.com:
One technique to avoid this system behavior is to delete the Aperture Library file that gets installed by default into the User’s Pictures folder. However, if a user wants to use Aperture in lieu of (or in addition to) iPhoto, then this workaround solution is unsatisfactory. After a bit of experimentation, I came up with a solution that allows me to regain access to my desktop image options while retaining the ability to run Aperture: create the Aperture Library file onto another drive volume or folder outside the User’s Pictures folder.
Once you do so, your selector options in the Desktop & Screen Saver pane will return to normal.