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.

Muhammad Alnajlat

Light, fast and easy-to-use plugin for branding WordPress login page by setting your brand logo and theme color and extra simple styles.
What you can do with Brand My Login:
<ul>
<li>The simplest things you need to brand your login page.</li>
</ul>

Brand My Login

Brand and style your WordPress login page quickly and lightly.

wp Custom Login vs Brand My Login