In general, it’s safe to assume that someone has copyrights to any content you want to use.
It’s important to remember that the resources you create for use in your own classroom may not be okay for distributing on the internet. Therefore, you should be sure that you have the right to use anything you post on TPT. Using imagery, names, titles, and key phrases from well-known works or famous brands can be problematic, even if you’ve adapted your own version.
The safest approach to creating resources is to use only work (titles, text, images, photographs, graphics, and so on) that you’ve created yourself. If you’re using anything that you didn’t create completely yourself, you need to determine whether there’s a fair use exception, or get permission from the creator to use it in on TPT. Here are some links to information about fair use:
http://copyright.columbia.edu/copyright/fair-use/what-is-fair-use/
http://www.copyright.gov/fls/fl102.html
The major exception to this principle is work that has aged out of copyright protection and is in the “public domain.” It can be difficult to know whether a work has passed into the public domain, but there are some helpful resources out there. Check out this chart from Cornell.
If you want to check out more information on copyright and trademark topics, check out our Copyright & Trademark section in TPT U, and consider taking our Copyright & Trademark Quiz!