• An image you find on the Internet was taken or made by someone and that person should be given credit.
• Because an image exists in a fixed form (unlike an idea), it is protected by copyright.
• Images don't need a copyright symbol (a "c" inside a circle) to be protected by copyright.
• Your best bet is to search for images the author has given permission for others to use (under a Creative Commons license) and to credit the author.

• search Flickr's Creative Commons. Photographers here have specified how their images may be used.
• search Wikimedia Commons, where again, photographers are identified and have specified how their images may be used
  
• the Library of Congress' American Memory and Digital Collectionshttp://www.loc.gov/library/libarch-digital.html is a treasure trove of primary source materials--photographs, films, recordings, etc. Some--but not all--of the collection is in the public domain (not protected by copyright) because it belongs to the American people (Public Works projects) or is old enough to have fallen out of copyright. Works are clearly identified so that you have the information needed to cite it.
  
• use Google's "Advanced Image Search." Under "Usage Rights" select "Only images labeled for reuse." Once you have found an image, click on "Website for this image" on the right sidebar so that you can find the source and author of this image should you need to cite it later.
• "Finding and Using Public Domain Photographs"http://www.publicdomainsherpa.com/public-domain-photographs.html from Public Domain Sherpa

• Citing Primary Sources from the Library of Congress Digital Collections (MLA style)
• Son of Citation Machine's Web-Based Media entry
• Easybib's Digital Image entry
• How to Cite Other Common Sources from Purdue OWL