$(document).ready(function() {
	$('.thumbnail').click(function() {
		var mainPicture = $('#mainPicture').attr('src');
		var thumbPicture = $(this).attr('src');
		$(this).attr('src', function() { return mainPicture; });
		$('#mainPicture').attr('src', function() { return thumbPicture; });
	});
});