Create A Upload Manager For Texteditor?

I'm trying to create a upload manager for my texteditor. In my javascript I call a image uploadbox like this:

Code JavaScript:

if (cmd == "insertimage" && !value) {
var imageUrl = PopupCenter('http://www.mysite.com/imageupload.php', 'Upload',400,400);
if (imageUrl) {

[code].....

But I'm not sure how to do so... If I just for the ease of it make a inputfield in my imageupload.php like this:

Code HTML4Strict:

<form action="" method="get">
<input name="imageurl" type="text" />
<input name="Submit" type="submit" />
</form>

How do I pass the value back on submit?