Broken Image links

All image links inside archive are broken, because they only create a relative link to the image itself and not an attachment. For Gitlab I created a fix: update.py:fix_images_links(), but the propper fix need to take place in textileconverter.py, as archive use this method too.

@enrico: Don't know if it is worth fixing it. I just wanted to document it, that this is broken and that we have a fix already.