Sometimes randomly rotating your desktop wallpaper through the Desktop System Preferences pane is just a little overkill. Wouldn't it be nice if you had a simple drag-and-drop Automator action for setting your own wallpaper whenever your heart desired a change of pace? Then why not use this macosxhints post to make one. A few simple steps creates an action that you save as an app somewhere (I named my action 'Wallpaper') and simply link to in your Dock. For added convenience I saved it as a Finder plugin so I can right-click any picture I find to quickly chose Automator > Wallpaper from the context menu. Handy.
As an extra tip: make sure you turn off automatic wallpaper rotation in the Desktop Preferences pane if it's on. Otherwise, at least from my testing, this action won't work.