If an item (photograph, video, file) does not explicitly state that you can use it, you should assume you do not have permission and need to request it. Getting permissions is not always expensive (or necessarily cost anything).
When you find a photograph, look for the image data to see if it is tagged with a Creative Commons license or some other wording that states you can reuse without asking.
[One man looks on as another man prepares Univac computer to predict a winning horse] / World Telegram & Sun photo by Herman Hiller.
Look at the metadata (information about the photograph):
Check the Library of Congress page about Copyright and other Restrictions to see what "No Copyright Restrictions Known" means for reuse:
With this information we need to decide whether we feel like we can use this image. In addition to the above statement that their are no known restrictions on the image, our image states that the original rights were transferred to the Library of Congress. Under U.S. copyright law, U.S. Government works are not protected by copyright.
Any images should credit the original source whether they are in the public domain, licensed through Creative Commons, or under copyright. See the Crediting Images tab for more information.