If you would like System Preferences to set your desktop (like it normally is with most Macs), you can disable The Twenty-Four Hour Movie. You can later re-enable it any time you want to use its features.

Disabling the program:
  1. Open The Twenty-Four Hour Movie application.

  2. If the checkbox “Change the desktop picture depending on the time” is checked, uncheck it.

    You should see a window similar to this:

    The main winow with desktop picture changing disabled

Even though your desktop will not immediately change, the program is disabled and is no longer setting your desktop. You should now open the Desktop & Screen Saver preference pane to set your desktop to something else.