The Creative Commons portion of the image website Flickr is a terrific place to find images, and also participate in the sharing of these images. This first page shows you the various types of licenses and attributions under which photographers allow their images to be used.
If you are looking for images to illustrate a paper or blog, go to Flickr.com and click on the word ‘search’ in the upper right hand corner of the webpage. This takes you to a new search page. You will see a drop-down list on the left side of the search field. Click on that and, from the menu, choose The Commons, then type in a key word for flickr to search on.
You can download the picture to your desktop, but please pay attention to the licensing and copyright tags on the picture page.