This will prob be done using javascript and DHTML. I would expect that the window opened when the form is submitted and data passed to that window as follows:
<form name="form" method="post" action="uplaoder.php" target="WINDOW1" onsubmit="window.open('','WINDOW1','resizable=1,scrollbars=1,width=400,height=300')">
YOUR FORM
</form>
Your upload script would do its stuff and then close the window. The other page would monitor the window (by using the name WINDOW1) untill it has closed. Of course you would add the Javascript to change the background when the form is submitted, and when the upload window closes. I would post the code for you, but this is a PHP forum after all 🙂