From FCKeditor Docs

ImageUploadURL

This option defines the URL to post files through the "Upload Tab" in the image dialog. It can point to the default file uploader:

FCKConfig.ImageUploadURL = FCKConfig.BasePath + 'filemanager/connectors/' + _QuickUploadLanguage + '/upload.' + _QuickUploadExtension ;

or any custom implementation of it:

FCKConfig.ImageUploadURL = '/myuploader/upload.php';