In addition to "signing" the URL, you may also require users to login to view the original image, and serve visitors a compressed version. This could give you the benefit of gaining users (good for VC) while respecting the guests, as well as protecting your investments.

Back in the old days where everyone operates their own server, another thing you could do is to just setup a per-IP traffic throttling with iptables (`-m recent` or `-m hashlimit`). Just something to consider in case one day you might grow tired of Google Cloud Storage too ;)