replace all .click() functions with:

$('img.thumbs_img').click(function(){
        $('img.big_img').hide(); // Hides all big images
        var id = $(this).attr('id');
        id = str.replace("thumbs_img_", "big_img_");
        $('img#'+id).fadeIn('slow'); //Slowly fades in selected image
    });

replace all .click() functions with:

$('img.thumbs_img').click(function(){
        $('img.big_img').hide(); // Hides all big images
        var id = $(this).attr('id');
        id = id.replace("thumbs_img_", "big_img_");
        $('img#'+id).fadeIn('slow'); //Slowly fades in selected image
    });