If I changed my desktop background picture, that picture would also be set to login screen. Now it's not working. Whatever I change, login background is the same picture (Ubuntu's default picture) How can I make it work?
This question has been answered multiple times, here is an appropriate link.
On your behalf I have tried all answers in this thread on Ubuntu 16.10 and I have only had success with one. Unfortunately it's the ugly one.
Firstly, make sure the picture you want to use is in .png format, then open your terminal:
cd /usr/share/backgrounds sudo cp /directory/path/to/my/picture /usr/share/backgrounds sudo mv warty-final-ubuntu.png warty-final-ubuntu2.png sudo mv 'my-picture-name.png' warty-final-ubuntu.png
Then log out to see your new login wallpaper.