I used Ubuntu Tweak to change the login screen background, but it didnt work for my user. My Ubuntu has 2 users. To my user (admin) it shows the correct image (that had set in Ubuntu Tweak) for less than 1 second and change to my desktop wallpaper. To the other user (normal user) it shows the correct image. After login, it changes to the desktop wallpaper (another picture) correctly. Any idea? Thanks.