Use an AppleScript to Force a Desktop Picture Change
You may use the Mac OS X feature in the Desktop preferences to have your desktop images set to change every n minutes in random order. Sometimes the desktop will display an image that displayed earlier in the day, and you may want to skip over that image easily. The following AppleScript will cause the desktop image to change, in whatever order you have set, each time it is run. (The script assumes that you have set the picture to change every 30 minutes).
To maintain your current change interval, modify the change interval lines to reflect your interval, measured in seconds. Thus, for a 30-minute setting, you use 1801 and 1800 seconds. One hour would be 3601 and 3600, etc. For this script to work, you must have both Change Picture and Random Order enabled on the Desktop tab of the Desktop & Screen Saver System Preferences panel.
You can save the script as an application and leave it somewhere accessible so you can run it with a double-click.