Are you unhappy with the limited selection of time settings available in the Desktop prefs panel's "Change picture" options? What if you want a new picture every 10 minutes? It's not on the list. Luckily, it's easily changed...
Once you have enabled desktop rotation via the Desktop preference panel, open up ~/Library -> Preferences -> com.apple.desktop.plist in your favorite text editor. Look for:
<key>ChangeTime</key>
<string>some_number</string>
Change some_number, in seconds, to something you'd like to use. Changes take effect the next time you login.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030325020900756