Customize Login Screen of LinuxMint 17

I think the default LinuxMint login screen is not cool, need a new backgroud picture. How to change this backgroud picture? In LinuxMint 17 this is done by creating a new login "Theme" and use it. According to Custom Login Screen Background On Linux Mint 17 there is 3 steps to achieve this:

1. Create a new theme based on the default theme;

2. Pack this theme;

3. Set it as the login theme.

Firstly I choose a picture from Variety's Favorites folder as the new backgroud. Then run following commands:

sudo cp -r /usr/share/mdm/html-themes/Mint-X ~/StarWorld
cp ~/.config/variety/Favorites/fbdgbcb_-_imgur.jpg ~/StartWorld/bg.jpg
cp ~/.config/variety/Favorites/fbdgbcb_-_imgur.jpg ~/StartWorld/screenshot.jpg
# modify name & description in ~/StarWorld/theme.info
tar czf star-world.tar.gz ~/StarWorld

Next login to Xfce desktop, startup "Login Manager", add a new theme, choose star-world.tar.gz.

Now logout and see the effect.