I just found this tip on ubuntuforums.org which works for me:
http://ubuntuforums.org/showpost.php?p=8681086postcount=2
alt f2 launch run box
gconf-editor
Then navigate the tree to ( / apps nautilus preferences )
Make sure you have preferences clicked on.
Then on the right scroll down to
** Changed in: gnome-shell (Ubuntu)
Importance: Undecided = Low
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Gnome Shell isn't showing a wallpaper
https://bugs.launchpad.net/bugs/509951
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** Attachment added: Screenshot
http://launchpadlibrarian.net/38117580/Screenshot-18.png
** Attachment added: Dependencies.txt
http://launchpadlibrarian.net/38117581/Dependencies.txt
** Attachment added: XsessionErrors.txt
http://launchpadlibrarian.net/38117582/XsessionErrors.txt
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