var open_window = 0;
var top = 500;
var left = 400;

function init_popup()
{
	 if(screen.width)
	 {
	   top = screen.height / 2;
	   left = screen.height / 2;
	 }
}
function popup_window(url,width,height)
{
  if(open_window)
  {
   open_window.close();
  }
    open_window = window.open("preview.php?url="+url,"Bilderansicht", "width=" + width + ",height=" + height + ",left="+ left +",top=" + top);
}