function str_replace(search, replace, subject)
{
    return subject.split(search).join(replace);
}

$(".preview").live("mouseover", function()
{
    $(".preview").removeClass ('selected');
    $(this).addClass ('selected');
    newsrc = str_replace('.jpg', '_full.jpg', $(this)[0].src);
    newsrc = str_replace('.gif', '_full.gif', newsrc);
    newsrc = str_replace('.png', '_full.png', newsrc);
    $('.mainfoto')[0].src = newsrc;
});