<script>
function apripopup(nome,largh,altez) {
x = screen.availWidth;
y = screen.availHeight;
x = ((x / 2) - (largh / 2))
y = ((y / 2) - (altez / 2))
window.open(nome,'pippo','width=' + largh + ',height=' + altez + ',left=' + x + ',top=' + y + ',location=no,toolbar=no,menubar=no,scrollbars=no, resizable=no,status=no');
}


<a href="#" onclick="apripopup('prova.jpg',500,200);return false">