I noticed that if you have your desktop picture (wallpaper) set to shuffle between a number of images, and you don’t like the currently-selected picture, you can manually skip it by killing and restarting the Dock. Just type the following into Terminal:
killall Dock
When the Dock restarts, it will shuffle to another desktop picture.
[robg adds: I believe a "friendlier" version of this command is killall -QUIT Dock, which tells the Dock to quit nicely, instead of force quitting it. If you find yourself doing this regularly, you could turn it into a simple Automator application, using the above command, or the AppleScript alternative (tell application "Dock" to quit). Then again, if you're doing this a lot, perhaps you should look to change your collection of desktop images!]
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