jQuery(function($){

$('.gallery_small img').click(function(){
    var bigImgSrc = '../images/gallery/' + $(this).attr('src').split('/').reverse()[0];
    $('.big_gallery_img').fadeOut('slow', 
        function(){
            $('.big_gallery_img img').attr('src', bigImgSrc);
        }).fadeIn('slow');
    $('.gallery_small a').removeClass('active');
    $(this).parent('a').addClass('active');
});

});