This question already has an answer here:
I changed the default login screen wallpaper to a custom one as well as the desktop background to a another custom one(basically I have two different backgrounds one for login screen and one for desktop).
Unfortunately recently it started to act weird. At the startup the login screen wallpaper shows for like half a second and then it automatically changes to the desktop wallpaper before I even log in. I don't use anything else to change the backgrounds apart from unity tweak tool and to change the login screen I replaced the default file in /usr/share/backgrounds.
It's a minor problem but I cannot solve it myself and need some help.
Thanks in advance!