Disclaimer: The information presented below is a general opinion and is informational.
If you have additional questions you may want to consult an attorney
who practices in your part of the world or purchase a book
which covers this information in more detail
.
Copyright
It's Your Photo
You must own the copyright to your photo. Did you take the photo, for yourself? If so, you're OK. If you hired someone else to take it you should have a written agreement with them stating that you own the rights to the image. The same is true if you were being hired as the photographer.
Artwork
Does your photo include artwork or other copyrighted objects such as a Donald Duck statue? Pictures that feature other copyrighted items including paintings, murals, photos, sculptures and cartoon characters have their own copyright. This may extend to property photographs as well. To sell other copyrighted material with your material you would need a written release from the copyright owner to sell the image for commercial purposes. Editorial and other fair use purposes may be OK, and sometimes you can submit a photo for editorial purposes only. Generally speaking, it always best to avoid any question of violating other privacy, copyright or trademark laws.
Recommended reading: Books on Copyright Laws
People
Model Releases
Photo buyers often like people in a photographs. People convey messages and readily show action and interaction. But for non-editorial identifiable people in a photography you'll need a signed model release. Depending upon the buyer they may have their own forms or requirements for such forms.
Children
Minors must have a release who has been signed by a parent or other legal guardian.
Nudity
Nude models should look of a legal age and you will often be required to provide a copy of a photo ID which shows age.
Celebrities and Sports Teams
Sports teams, associations and events generally don't permit unauthorized commercial usage of images which identify the team. They can use trademark law (for logos and names) and invasion of privacy if it was a entry fee event. These organizations must vigorously enforce these rights or they can lose them. An excellent book for pre-made and court tested forms is: Professional Forms for Photographers
Summary
America derives a lot of our legal interpretation based upon case law. As higher courts interpret the law lower courts can cite that decision as part of an arguement. The net result is that "the law" will evolve over time. If you have any concerns about what is legal we recommend you consult an attorney or purchase recently published guides on copyright law.
If you are looking for Copyright forms the USA Federal Government publications can be found here.