An image is copyrighted as soon as it is created and generally the copyright in a photograph belongs to the person clicking the shutter, the photographer.
However it is sometimes difficult to prove that a particular photographer owns the copyright and this is where registering the image is important so that a photographer can protect their work. Most countries do not offer a means of registering a copyright but one exception is the US where a copyright can be registered with the US Copyright Registration Office for currently $55.
That might appear to be a lot of money to register a single image but it is possible to register a whole batch of images in the same registration all for the one payment of $55.
I tend to register my images once every few months prior to publication and there is no limit to the number that can be registered in one go, the only limit is an upload file of size of 500mb but you can upload as many files as you need to as long as they are each below 500mb.
The copyright office will accept the upload of compressed zip files so it is possible to register a large number of previously unpublished images under a single application.
The process of registration is very simlpe, you need to register a free account and then give details of the creator of the work, the owner of the copyright which is generally the same person unless it is work for hire in which case it is the employer and then a payment is made.
The application is reviewed currently in around three months and if there is no problem a copyright certificate will be granted dated from the date of the application. This means that as soon as you make the application you can start publishing the images knowing you have the protection of the copyright registration.
As well as being a method of proving that you own the copyright by registering your images you gain the protection of the courts in the US who can impose penalties ranging from $750 to $150,000 for each breach of copyright of any registered work.