Niaj Morshed

Just Install the plugin, then you can see the login form has changed, 
after this go to the Settings->custom-login & put your logo & background image link which you want. 
Enjoy.

wp Custom Login

This plugin can changes the logo & background image of the login form.

Harish Chouhan

Themeist Co

Custom Login Logo lets you replace the default WordPress login logo with your own — no need to touch code or upload files via FTP.
Just activate the plugin and use the built-in media uploader on the settings page to upload your custom login logo. It’s quick, simple, and works with most themes.
This plugin is perfect for branding client sites, personalising the login screen, or maintaining a consistent visual identity.
Features: 
– Upload your custom logo using the built-in WordPress media uploader 
– No need for FTP or coding 
– Works with most themes and setups 
– Retina/high-DPI image compatible 
– Lightweight and fast
If you find this plugin useful, please consider leaving a 5-star review on WordPress.org.

Custom Login Logo – Easily Add a Logo to Your WordPress Login Page

Easily add a custom logo to your WordPress login page using the built-in media uploader.

wp Custom Login vs Custom Login Logo – Easily Add a Logo to Your WordPress Login Page