As @tkohhh indicated: bad idea today.
Even if the only pictures you post in photostructure are images you intend to be available to the public (i.e. you have no privacy concerns), there are all kinds of functionality that you can’t disable and you don’t want to expose to the public, such as “shutdown server”, “settings”, “rebuild”, etc.
IF in spite of all of that, you still want to do this, (unofficially) it’s certainly possible today. Every picture in the library is assigned a unique asset id and the URL for that picture is therefore unique. It will look something like this: https://your.server.address:port/img/xxxx/actual (where xxxx is an asset id, and “actual” is for the original - there are URLs you can discover for various thumbnail sizes too). With some fancy firewall/reverse proxy rules that only allow a certain URL patterns you could conceivably make that secure enough.
Officially, you probably should open a feature request, and/or vote on the one that should allow for this (depending on how it’s implemented):