ecco prova con questo


codice:

NewWindow("TuaPopUp.asp","dataitem",'110','230');


// FUNZIONE PER APRIRE UNA NUOVA FINESTRA CENTRATA NELLO SCHERMO
function NewWindow(mypage, myname, h, w) {
	var winl = (screen.width - w) / 2;	
	var wint = (screen.height - h) / 2;
	winprops = 'height='+h+',width='+w+',top='+wint+',left='+winl+',hotkeys=no ,scrollbars=no,toolbar=no ,alwaysRaised=yes ,personalbar=yes';
	win = window.open(mypage, myname, winprops)

	if (parseInt(navigator.appVersion) >= 4) {
	win.window.focus();
	}



win.document.write('<html><title>'+alt+'</title><body bgcolor="#FFFFFF" leftmargin="0" topmargin="0" marginheight="0" marginwidth="0" onBlur="self.close()">'); 
	win.document.write('[img]+imageName+[/img]'); 
	win.document.write('</body></html>');
	win.document.close();


}