Custom Login Image as a banner

As the Introduction about the "Custom Login Image" on Usage, it shows we could put a banner as the image on login page. It doesnt seem updated as same as the version we have right now. Although I have the custom CSS on "Custom Login CSS" for what we want, it is not working well as not close enough by your setting. Please find the enclosed capture for your information.

Can anyone please tell me how to make it as the banner as we want?

Many thanks!