To find images that you can use in any publication for free, you will want to look for royalty-free or public domain images.
Royalty-free refers to the right to use copyrighted material or intellectual property without the need to pay royalties or license fees. This means you may be able to use these copyrighted works for free.
Similarly, images in the public domain do not have a copyright; this includes older works with expired copyrights. Images in the public domain may also be used for free.
Although these images may not require you to pay for their reuse, you still need to read any terms and conditions associated with their use. For example, does the creator or archivist of the image want to be cited? (Probably.) And, if you're planning on manipulating or changing the image, is that allowed by the creator? (Be sure to read!)
Photograph of Kennedy Family with Dogs During a Weekend at Hyannisport, 08/14/1963, attributed to Cecil Stoughton, U.S. National Archives, https://www.flickr.com/photos/35740357@N03/5589770246 and http://arcweb.archives.gov/arc/action/ExternalIdSearch?id=194258.
Puppy field evaluations by Renee V, some rights reserved, https://www.flickr.com/photos/reneeviehmann/4827604577.
Police Dog, Tess, 29/1/35 by Sam Hood, State Library of New South Wales, http://www.flickr.com/photos/statelibraryofnsw/3210838977/
Creative Commons licenses help creators retain copyright while allowing others to copy, distribute, and make some uses of their work, at least non-commercially.
All of the Creative Commons licenses require that users provide attribution to the creator when the material is used and shared. Some licensors choose the attribution requirement as the only condition for reuse of the material; this is designated as the BY license. The other five licenses combine the BY license with one or more of three additional license elements: NonCommercial (NC), which prohibits commercial use of the material; NoDerivatives (ND), which prohibits the sharing of adaptations of the material; and ShareAlike (SA), which requires adaptations of the material be released under the same license. (Adapted from http://wiki.creativecommons.org/ FAQ#What_are_Creative_Commons_licenses.3F.)
Use the Creative Commons search options to find images that may have these more permissive licenses that allow for image reuse. However, as it says on the Creative Commons page, with any image you find, "you should always verify that the work is actually under a CC license" and "if you are in doubt you should contact the copyright holder directly, or try to contact the site where you found the content."