Change or drop hero image

[chalin]

Originally posted by @lukpueh in #123 (review):

... the background image (which, IMO, makes the site look better, but that's an unrelated concern).

[chalin]

@lukpueh - if you (and other TUF stakeholders) can agree to and propose a suitable image, that would be great. Or, if you prefer that we just drop the image, that can easily be done.