Copyright is now automatic. If you take a photo it has copyright. You no longer need to register the work, include the copyright symbol, or deposit a copy to get copyright.
Now, if you want to sue for infringement, you will need to register the work. You can register your works online here.
Copyright lasts for a very very long time. If you own the copyright to a work, the copyright lasts 70 years after your death. If the copyright is owned by a company or organization, it last for 95 years after the work was created.
Blame Sonny Bono and Disney (and Victor Hugo, he knows what he did)
If you take a straight on photo of a mural in broad daylight with no people, birds, traffic, JUST the mural, that can not be copyrighted and is possibly copyright infringement.
However, if you take a photo of the mural at dusk with the street light illuminating part of the mural, that shows "creative expression".
The Copyright Office makes this clear in 909.3(A) (page 22) of the linked document below.
This is illustrated in the images below. The first image would be a decent photo for the 3D print designer who created the dinos, whereas the second photo includes "creative expression" in the arrangement of the dinos.
The Copyright Office is looking into the nitty gritty of how AI affects something's copyrightability. The Copyright Office (and the Courts) have already issued decisions regarding the ability to copyright something that is entirely AI generated.
There are several Creative Commons Licenses if you want to not use traditional copyright or make certain types of uses explicitly fine. All but CC-0 retain some element of control over how the work can be used.
The six common licenses range from CC-BY which retains only that you must be cited as the original work, to CC BY-NC-ND, where you must be cited as the creator of the original work, no derivatives can be made of your work, and it cannot be used for commercial purposes.
You can release your images into the public domain using the CC-0 license. This is the license that retains no rights.
Other things that are in the public domain (non-exhaustive)
Once a work is released into the public domain, YOU HAVE NO CONTROL OVER IT. Derivative works that you don't approve of can be made. Getty can charge an arm and a leg for it, and you cannot stop them.*
*Carol Highsmith sued Getty Images after finding they were licensing her images AFTER she had made them public domain. The judge agreed with Getty that as she had given up her rights to them, she could not stop Getty from charging people to license them. <source>
Getty Images still tries to license public domain images, because that's not illegal. Just always remember that if you want to use an image, check to see if it's freely available instead!
Selected images Getty wants you to pay for: photos by Patricia Highsmith, Dorthea Lange's photos that were funded by Resettlement Adminstration/Farm Security Administration (during the Great Depression) and the War Relocation Authority (during WWII), and NASA photos.
All of these are public domain. Download them for free.
Fair Use is your friend!
Fair Use is how you can use a piece of work that is not in the public domain.
Fair Use is determined by 4 Factors:
"(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work. "
As stated in 17 U.S.C. 107 - Limitations on exclusive rights: Fair use.
If you are commissioned to take some photos, the copyright situation can be different. Whether you or the commissioner is the copyright owner depends on the contract the two of you decide on. For more information, please see the Copyright Office Circular on the topic linked below.
Copyright is not your only concern.
Trademark is also a concern when you are taking photos of branded works. Among other things, Disney is using trademark protections on the original Mickey from Steamboat Willie, which entered the public domain January first this year. You may have problems selling works wherein company's trademarks appear.
People may not want their likeness used for anything public/commercial, and while you may win a lawsuit on first amendment rights, it would be easier to just get people's permission in your photographs. <related>