Almost everything is copyrighted and requires permission from its author for most uses. There are, however, some exceptions.
Some media have been made available for anyone to use, waiving the need to obtain direct permission from the author. For example, a common license is an "Attribution" license, meaning that you can use the media for any purpose as long as you give credit to the creator somewhere in your work.
Creative Commons offers a variety of attribution licenses that creators can apply to their work for public use. Learn more about Creative Commons licensing. Search for works with a Creative Commons license.