Add the following to your theme’s functions.php file. Also remember to add your login logo (png) into your theme’s “images” folder.

function custom_login_logo() {
        echo '';
}
add_action('login_head', 'custom_login_logo'); 

function my_login_logo_url() {
    return get_bloginfo( 'url' );
}
add_filter( 'login_headerurl', 'my_login_logo_url' );

function my_login_logo_url_title() {
    return get_bloginfo('name');
}
add_filter( 'login_headertitle', 'my_login_logo_url_title' );