CKEditor, Image Upload (filebrowserUploadUrl)

May be it's too late. Your code is correct so please check again your url in filebrowserUploadUrl

CKEDITOR.replace( 'editor1', {
    filebrowserUploadUrl: "upload/upload.php" 
} );

And the Upload.php file

if (file_exists("images/" . $_FILES["upload"]["name"]))
{
 echo $_FILES["upload"]["name"] . " already exists. ";
}
else
{
 move_uploaded_file($_FILES["upload"]["tmp_name"],
 "images/" . $_FILES["upload"]["name"]);
 echo "Stored in: " . "images/" . $_FILES["upload"]["name"];
}

The URL should point to your own custom filebrowser url you might have.

I have already done this in one of my projects, and I have posted a tutorial on this topic on my blog

http://www.mixedwaves.com/2010/02/integrating-fckeditor-filemanager-in-ckeditor/

The tutorial gives a step by step instructions about how to integrate the inbuilt FileBrowser of FCKEditor in CKEditor, if you don't want to make our own. Its pretty simple.