Closing A Popup Before Opening Another

I need some help with closing a popup window. I'm making an online portfolio,
that has one base page with text and thumbnails. When a user clicks on a thumb,
a new window opens containing a larger image and a caption (each contained on a
separate htm document). I do this with a function in the header script as follows:

var picpage=" ";
var params=" ";

function fullpicwin(picpage,params)
{
window.status="Opening picture... please wait";
window.open(picpage,'fullpic',params)
}

picpage (the name of the htm file), and params (window size, etc.) are passed
from the click code by each thumb. Each popup is sized differently, of course,
based on the side of the larger photo. All controls, etc., in the popup are
turned off. The user can click the "x" box or a form button at the bottom of
the popup to close the window.

All this works very nicely... EXCEPT...

The user can move the mouse back to the main page and open ANOTHER popup by
clicking on another thumb BEFORE closing the existing popup. The problem is the
2nd popup retains the size of the first, so of course the photo may be blocked
by the borders, etc.

What I want to do is force the closing of the first popup if the user tries to
open a 2nd and so on. There is no reason to have more than one large photo
showing at the same time.

I thought I could put some code in the function that checks for the existence
of a window named "fullpic", and if it's there, closes it first, then opens
another. But I can't seem to get the syntax right.