Login Background CSS -- Ultimate Branding

https://premium.wpmudev.org/blog/customize-wordpress-login/

I am using Ultimate Branding plugin and have used the above article for Login CSS reference.

I want to add a custom background image to my login screen and I found the css below in the article:

body.login {
background: #fbfbfb url('your-bg.jpg') no-repeat fixed center;
}

Where do I put 'your-bg.jpg' (or whatever I change my background image file name to)? I have FTP access, but not really sure where I should place the file.

Please respond as soon as possible. Thanks in advance.