var opened = 0;
function PopUp(I, W, H){  
  if ((navigator.appName == "Microsoft Internet Explorer" ) &&
    ( navigator.appVersion.substring(0, 2) >= 4.0)){
    if (opened==1) {
      janfoto.close();
      opened=0; 
    }
    if (opened==0){
      janfoto = window.open(I,'_blank','scrollbars=yes,width='+W+',height='+H+',top=50,left=100');
      opened=1; 
    }
  }
  if ((navigator.appName == "Netscape" ) &&
    ( navigator.appVersion.substring(0, 2) >= 3.0)){ 
    janfoto = window.open(I,'_blank','scrollbars=yes,width=340,height=260').focus();
  }
}