Just because an image is found on the internet, it doesn't mean that it is there for you to use without permission.
Although student work can often be considered Fair Use of a copyrighted work, it is good practice to always look for a statement of permissible use and/or a suggested credit line before using the image.
Can't find any? If possible, contact the copyright holder to ask permission (hey, it never hurts!), or failing that, find an alternative resource.
"The World Digital Library (WDL) is a project of the U.S. Library of Congress, carried out with the support of the United Nations Educational, Cultural and Scientific Organization (UNESCO), and in cooperation with libraries, archives, museums, educational institutions, and international organizations from around the world." -from World Digital Library website
"The Web Gallery of Art is a searchable database of European fine arts and architecture (3rd-19th centuries), currently containing over 48.600 reproductions. Artist biographies, commentaries, guided tours, period music, catalogue, free postcard and mobile services are provided." -taken from Web Gallery of Art website
"The Internet Archive, a 501(c)(3) non-profit, is building a digital library of Internet sites and other cultural artifacts in digital form. Like a paper library, we provide free access to researchers, historians, scholars, the print disabled, and the general public. Our mission is to provide Universal Access to All Knowledge." -from Internet Archive website
