Many Flickr users have chosen to offer their work under a Creative Commons license, and you can browse or search through content under each type of license

Some institutes such as The Library Of Congress, Powerhouse Museum, Brooklyn Museum, Smithsonian Institution, and others can post and share images that have no known copyright restrictions.

Make sure, especially with images from Flickr that you check the license is the appropriate  for you if you are going to use someones work from this site.