If you want to change default wordpress logo on the wp-login.php page then you can change wordpress logo by your logo by just adding following lines in your theme’s function php files.

function my_custom_login_logo() {
    echo '<style type="text/css">
        h1 a { background-image:url('.get_bloginfo('template_url').'/images/login-logo.png) !important; }

add_action('login_head', 'my_custom_login_logo');

You have to upload your logo in your theme’s directory with name “login-logo.png