System Preferences does not include any easy way to reset all desktop backgrounds throughout Mission Control; instead, each is changed individually. This hint offers an easier workaround than resetting each one-by-one or deleting and restoring them all.
In order to change all wallpapers, go to Desktop 1 and change its wallpaper through System Preferences > Desktop & Screen Saver. Then enter:
killall Dock in Terminal.
This will set all desktops to the same wallpaper.
The one caveat I've noticed: if you previously did set a wallpaper for an individual desktop, it will not change. This hint seems to only apply to desktops created through the plus symbol in Mission Control.
[crarko adds: I had a bit of difficulty making this work until I removed my existing Spaces (other than the first) and re-created them using the Mission Control plus sign, as mentioned in the hint.]
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Mac OS X Hints
http://hints.macworld.com/article.php?story=20111001130027239