Simply citing an image is not enough. In most cases, you must make sure that you have permission to use it. This is because an image is an entire work, and the person who created it has the sole rights to its use.
Different permissions are used: Attribution, Share Alike, and Noncommerical, to name a few. Not sure what these mean? License definitions are available on the Creative Commons website (scroll down to see a list of the licenses and their descriptions).
Use this guide to locate images which you can reuse. And when you use them, be sure to include the citations!
You are free to use images that fall into the public domain without seeking permission. Terms vary by country and when the image was created, but for works created in the United States the general rules are the life of the creator +70 years for unpublished works and 90 years from the original publication date for any published works. This handy copyright term chart created by Cornell University breaks down all the exceptions to these rules.
When using the work of others, another concept to keep in mind is fair use. Fair use is an exception to U.S. copyright law that allows you to use copyrighted materials (including images) without permission if certain criteria are met. The law states the following:
The fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.