How To Change Login Screen Background On 11.10 [duplicate]
It's in the file /etc/lightdm/unity-greeter.conf. Open it with gksu gedit /etc/lightdm/unity-greeter.conf from a terminal or run dialogue. Set background= to whatever file you would like.
However make sure that file is accessible to all. It can not be in an encrypted home folder or such. The problem you have is most likely due to unity-greeter for some reason can't access the file you chose.