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.

Scott DeLuzio

AMP-MODE

WordPress Login Logo Editor is a plugin that easily lets you edit the form and logo shown when logging into your WordPress site. WordPress Login Logo Editor allows you to edit:
<ul>
<li>The image above the login form on your WordPress site.</li>
<li>The URL that a user gets directed to when the logo is clicked. By default you are directed to http://wordpress.org.</li>
<li>The title text that appears when your mouse hovers over the login logo. By default this text says Powered by WordPress.</li>
<li>Style the login window with custom CSS.</li>
</ul>

Login Logo Editor

Easily lets you edit the form and logo shown when logging into your WordPress site.

wp Custom Login vs Login Logo Editor