Can't change the login background

I recently installed Ubuntu 12.04.
Using ubuntu-tweak I changed my login background to "Twilight_Frost" which is shipped with default installation.
My desktop background was different from login background.

Things were working fine but suddenly one day the login screen is set same as the desktop background.
The 'Twilight_Frost' background is displayed for a split second and then overriden by my desktop backgound on the login screen.

I am not sure what change has lead to this. Maybe some updates which Ubuntu prompted my or something else, no clue.

The ubuntu-tweak window still shows the login background different but it is not working for me any more :(

Anyone, any clue ?? Which logs to check ?
I have checked couple of threads regarding this, some were saying read permissions etc but nothing is working on my machine