Copywrong, Copyleft, Copyright
There's a lot of confusion among photographers and the public at large regarding the ins and outs of copyrighting photographs. Keeping in mind I'm not a lawyer, here's my version of a short introduction to the topic.
First of all, there's a big myth that I need to dispel up front. Contrary to popular belief, and against what some folks may wish were true, everything on the internet is not in the public domain. Just because you can browse the web for free, doesn't automatically give you permission to take what you find for your own use.
Consider this analogy. Suppose you are walking down the street and see a billboard featuring an image you really like. It's in public view, but few would consider it ethical to climb up on a ladder and abscond with that image. Clearly, that would be stealing. But if that same image was posted to a website, at least some people would consider it perfectly OK to copy for their own use. Yes, in the latter case, you would only be making a copy while it would be necessary to deface that billboard to take the image, but conceptually both are the same. Any image worth displaying in either medium represents the hard work and skill of some photographer somewhere. To take it without permission would be wrong.
In the United States, images are legally copyrighted the moment you press the shutter release. You don't have to do anything to enjoy the protection of copyright law. Every image you see is copyrighted and can't be used by others without express permission. This holds true even when no copyright notice accompanies the display of an image.
There does exist a concept known as "copyleft" though that takes the idea of copyright and flips it on its head. More common with software licenses, copyleft grants the user full permission to distribute covered works or repurpose them for derived works. All copyleft terms that I am aware of do stipulate that any copies or derived works retain the same copyleft terms. That means, for instance, you wouldn't be able to sell copies since you didn't have to pay for it either. Since this is not the default though, the legal copyright owner must clearly grant such permissions. The GNU General Public License (GPL) and some variants of Creative Commons licensing are considered copyleft licenses. The copyleft notice always must stay with the covered work to comply with the terms and to ensure that new versions retain the terms.
So, back to actual copyright. As I said, you are protected by copyright law automatically as the author (photographer) of an image. The moment it is "fixed" in a tangible form, it's covered by copyright. And that includes the copy written to your camera's memory card. You don't have to file any forms or submit any paperwork.
But, that's not the full bottom line. There are significant legal advantages to filing. If anyone does steal your image, you can only get statutory damages unless you registered your copyright with the U.S. Copyright Office before the infringement occurred. And although you would generally get reimbursed by the plaintiff should you win, you would need to pay any needed lawyer costs yourself upfront. If you register up front, many lawyers will take the case on spec since the case would be relatively cut and dried, and you can get punitive damages on top of the statutory damages of up to $150,000 per infringement.
Filing your copyright registration is easy. Back in the days of film, we use to have to scan copies of every slide and burn the results to a CD, then mail the whole packet in with a check or money order. In the digital world, everything is already in a form that it can be filed. And the Copyright Office has a new electronic Copyright Office (eCO) online portal. How cool is that? If this would be your first time registering a copyright, you'll need to create an account. To begin the process, go to http://copyright.gov/eco/ and follow the instructions. You can download an Acrobat pdf file with all the gory details from https://www.copyright.gov/fls/fl107.pdf
It is preferable to file the registration before you publish the images as this simplifies the process, but better late than never. Registering a batch of images requires that you give a title to the "collection." The simplest solution to this dilemma is simply to name it "Photographs taken by you during such and such period of time." How frequently you batch things up and file depends largely on how much you shoot. Published images can't be filed together with unpublished images.
As I said at the outset, I'm not an attorney. But if you do find yourself in need of more technical advice, one good place to turn to is http://www.photoattorney.com, the online home of Carolyn E. Wright, a full-time attorney who specializes in legal help for photographers. She also happens to be a pretty good photographer in her own right.