Use of images in student work and for educational purposes is considered fair use by the Visual Resources Association. All images should be properly cited with information on the copyright holder, if known. For more information about the fair use of images for educational purposes see the VRA's Statement on the Fair Use of Images for Teaching, Research, and Study.
Also, see the College Art Association's page "Intellectual Property Rights and the Arts".
The Picture Licensing Universal System (PLUS) Coalition strives to "simplify and facilitate the communication and management of image rights".
For more information see the page for Dickinson's Visual Resources Center.
Creative Commons licenses allow creators to license their work and specify how they allow others to use it before putting it online. Google and other sites, such as Flickr, allow you to search for specific types of licenses or rights.