Comment 1 for bug 1410770

This code have been move to greeterbackground.c. "greeter_background_save_xroot()" used to save background:
http://bazaar.launchpad.net/~lightdm-gtk-greeter-team/lightdm-gtk-greeter/trunk/view/295/src/lightdm-gtk-greeter.c#L1132
This line was accidentally deleted in 1.8.6 and restored in 1.9.0

There is a lot of changes in this branch:
https://code.launchpad.net/~kalgasnik/lightdm-gtk-greeter/background-transition
You can check if this error is still present in this branch.