In the US the second you "make" a photograph it is automatically copyrighted to you. About 50 some yrs ago they changed the law so that you did not have to register it in order to obtain an official copyright. Now the copyright is automatically granted to you as the "inventor". Also, an image does not need to say "copyright" in order to claim copyright infringement if someone else uses your image without permission. So right now all your images are 100% copyrighted and are exclusively your property...assuming you are living in the US. This also applies to anything you write, too.
This is true, as far as it goes. However, come time of a dispute, it is still your word against his. You need evidence. Registration provides that evidence, of your claim, and the date. I can register your photo too, and the earliest claim has a big advantage.
If you want to go to court to settle damages, registration is absolutely required (just to be admitted in court).
If you have material of monetary value, enough to be of concern, you damn sure better get it registered.
In the USA, that is the Library of Congress U.S. Copyright Office
See upper left corner, Basics and FAQ. It is all there, and easy to read.