Lifehacker has a splendid post on how to find cheap or free photos that you can use in your blogs, web designs and more.
Some of the tips include using a Creative Commons search interface, user-edited Wikimedia Commons, CCHits and CCMixter, EveryStockPhoto and some Google ingenuity by using the search term “this work is licensed under a Creative Commons.”
One of my favorite tricks not mentioned in the post is to search for content in Google using “site:.gov” to retrieve only government sites. Anything the United States government (including state, county and local entities) publishes is free reign; you’ve already payed for it! This works great for finding things like space photos, satellite images and useful Excel files.