The easy way to change your login screen background in Ubuntu

Open Application > Accessories > Terminal.


root@burhan07:~$ cd /usr/share/gdm/themes/Human

root@burhan07:/usr/share/gdm/themes/Human$ ls -l
total 624
-rw-r–r– 1 root root 387387 2009-12-16 06:33 background.png

Step 1. Create a back up for your background image (if you need it).
root@burhan07:/usr/share/gdm/themes/Human$ sudo cp background.png background.png.backup
root@burhan07:/usr/share/gdm/themes/Human$

Step 2. Go to your Picture directory and copy the picture to theme directory to replace the theme background picture.
root@burhan07:~/MyData/Pictures/Wallpaper Mac4Linux$ sudo cp Mac-OS-X-Tiger-2-1680×1050.jpg /usr/share/gdm/themes/Human/background.png

Your old file picture file name: Mac-OS-X-Tiger-2-1680×1050.jpg
Your new file name: background.png
The sharpness of the picture will be depend on the pixel size.
That’s it.
Now, you can log out to see your new login screen.