Artists, photographers, and other media content creators have rights to their works, just like authors do to their articles and books. So you need to make sure that the images and pictures you search can be used both ethically and legally for your projects. This is called Public Domain images or works that are Creative Commons licensed. For more information visit the digital writing 101 blog.
When in doubt, assume full copyright and find another image.
Always cite the source, regardless of where you found it and who's it is.
Baumel, A. (2010). Cholera treatment center in Haiti [Online image]. Retrieved October 2, 2010 from https://www.doctorswithoutborders.org
Flu epidemic [Online image]. (1919). Retrieved November 6, 2010 from http://www.history.net/photo/flu-epidemic-art/collections
No Author/Creator or Title or Date:
[Untitled illustration of a sleeping dog]. Retrieved December 5, 2010 Retrieved from http://www.sleepinganimals/pix.com