function imagePreview()
{
    var Images = document.getElementsByTagName('img');
    for (var i = 0; i < Images.length; i++)
        if (Images[i].className == 'nb_thumb_big')
        {
            if (Images[i].height > 0)
            {
                Images[i].style.marginTop = '-' + (Images[i].height / 2 - 40) + 'px';
                Images[i].style.marginLeft = '-' + (Images[i].width / 2 - 40) + 'px';
            }
            else
            {
                var ImgTemp = new Image();
                ImgTemp.src = Images[i].src;   
                Images[i].style.marginTop = '-' + (ImgTemp.height / 2 - 40) + 'px';
                Images[i].style.marginLeft = '-' + (ImgTemp.width / 2 - 40) + 'px';
            }
        }    
	
	
};