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.

George Pattichis

Simple Custom Login Page is a simple but robust plugin to customise the login screen of your WordPress website. You can change the logo, the link and the colors (background, text, links) of the login page to match your brand’s identity.
Considering all the bloated plugins and resource intensive implementations floating around, this lightweight and efficient solution is a must-have for all WordPress users/developers. It is also regularly updated to be compatible with the latest versions of WordPress and PHP.
<h3>Features</h3>
<ul>
<li>You can change login logo image</li>
<li>You can change login logo link</li>
<li>You can change page background color</li>
<li>You can change form background color</li>
<li>You can change label text color</li>
<li>You can change links text color</li>
<li>Changes are also applied to the forgot-password form</li>
<li>Changes are also applied to the notification messages</li>
</ul>

Simple Custom Login Page

A simple, lightweight plugin to easily customise the admin login page with your brand's logo and colors.

wp Custom Login vs Simple Custom Login Page