The Earth Seen From Apollo 17
It's an example of a great photo that is available for free from Wikimedia Commons. You are free to use it because it is in the public domain, as explained on the site:
"This file is in the public domain in the United States because it was solely created by NASA. NASA copyright policy states that "NASA material is not protected by copyright unless noted....
"This work is in the public domain because it was published in the United States between 1923 and 1977 and without a copyright notice. Unless its author has been dead for several years, it is copyrighted in jurisdictions that do not apply the rule of the shorter term for US works, such as Canada (50 p.m.a.), Mainland China (50 p.m.a., not Hong Kong or Macao), Germany (70 p.m.a.), Mexico (100 p.m.a.), Switzerland (70 p.m.a.), and other countries with individual treaties."
