<button class="btn btn-lg btn-primary btn-block enter" type="submit">
    Войти
    <img src="{{ asset('img/loading.gif') }}" class="img-loader" width="16" height="16" style="display: none;" alt="">
</button>

(function($, undefined){
    $('.enter').on('submit', function(){
        $('.img-loader').css('display', 'inline');
        $('.enter').prop('disabled', 'disabled');
    });
})(jQuery);