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.

António Campos

With this plugin you can easily choose one image from your gallery and use it as logo in your wp-login.php

AC Change Login Image

This plugin changes the image in the login page (wp-login.php), you can choose any image from your gallery and use it as login logo.

wp Custom Login vs AC Change Login Image