I see so many blog posts every day with a lot of tired clip art and see presentations that often use well-worn visuals. One of the things that Flickr does really well is aggregate lots of imagery that you are free to use.
A case in point is the photo documentary Documerica Project (1971-1977), the Environmental Protection Agency (EPA) hired a 100 photographers to go around the US and capture images relating to both environmental issues and everyday life within the US.
All of these pictures are freely available on Flickr as part of their commons initiative. You have museums, the US department of defense, NASA to name but a few organisations. In addition, thousands of people like me have included photographs under a creative commons licence.
A hippy and a puppy, what’s not to like?