thanks for your input. Yes, absolute links are the solution for the moment but they're not the smartest solution for us because the trouble begins when you start cloning the whole moodle installation for testing or whatever purpose.
The reason why we use our own upload folder ist the massive count of images and files we provide. So it's a lot easier to upload them via FTP than via Moodle's uploader. Also, we don't always want to provide all the information you have to enter while uploading via Moodle uploader.
But is there a specific (security?) reason why the moodle app can't handle relative non-moodle-URLs or could it be possible to add this feature in the future?