You don't need to register it or add a watermark or the copyright symbol to it.
It's a nice practice for people to do it on Steemit, but agree it's not required. Unless you talk about the picture you're posting such as look at this picture at such and such location. You could be lying, but think it helps give clarity it's your picture. Also, doing it is more for clarity that it's your picture than claiming "ownership" of the picture. Just my thoughts though.
You are absolutely correct :)