Changes the login logo to a theme based one.

wp_custom_login.php

<?php
/**
 * Changes the login logo and sets the appropiate sizes
 */
function custom_login_logo() {
	$path = '/images/logo_login.png';
	$size = getimagesize( get_template_directory() . $path );
	if ( $size ) :
		echo '<style type="text/css">
			.login h1 a  { 
				background-image: url(' . get_bloginfo('template_directory') . $path .'); 
				background-size: ' . $size[0] . 'px ' . $size[1] . 'px ;
				height: ' . $size[1] . 'px; 
			}
		</style>';
	endif;
}
add_action('login_head', 'custom_login_logo');
?>