Is the WordPress logo by default, this changes the file path of that image. Change file path and file name to your own needs.
function custom_login_logo() {
echo '<style type="text/css">h1 a { background: url('.get_bloginfo('template_directory').'/images/logo-login.gif) 50% 50% no-repeat !important; }</style>';
}
add_action('login_head', 'custom_login_logo');