function popup (img, ventana, ancho,alto){

   w = screen.availWidth;
   h = screen.availHeight;
		
		var popW = ancho, popH = alto;
		
		var leftPos = (w-popW)/2, topPos = (h-popH)/2;
		
		window.open(img,ventana,'width=' + popW + ',height=' + popH + ',top=' + topPos + ',left=' + leftPos);
}