Well, an alternative approach for their bare metal boxes is to request a KVM and link whatever image you in the KVM request form. They will then download it, put it into a USB stick, and plug in the KVM and USB stick. In my experience, you will have it all set up within the hour.

This is of course easier (although the KVM software is a heap of proprietary arse with a web interface or ancient Java blob), but if you are truly paranoid you will have to trust the Hetzner staff and their software stack not to bring something undesired along with your provided image.