• Just type the following into Terminal killall Dock When the Dock restarts, it will change to another desktop picture.
• If you find yourself doing this regularly, you could turn it into a simple Automator application

  tell application "System Events"
  set rotinterval to change interval of current desktop
  set change interval of current desktop to 1
  delay 1
  set change interval of current desktop to rotinterval
  end tell
  

  You can assign this script to a hotkey (using QuickSilver).

Quckly switch to next wallpaper picture is a post from: My MacOS X