var n = 6;
var f = false;
var b = new Array;
function prel()
{
   	if(document.images) 
	{
		  for(i=1;i<=n;i++)
            {
			   str = 'images/p' + i + '_o.jpg';
               b[i-1] = new Image();
               b[i-1].src = str;
			}
			f = true;
 	}

}
function on(im)
{
	var iim = im.substring(1);
	if (document.images && (f == true)) 
		{document.images[im].src = b[iim-1].src;}
}
function off(im)
{
	if (document.images && (f == true))
		{document.images[im].src = "images/"+im+".jpg";}
}