MLA Handbook by 
Publication manual of the American Psychological Association
The Chicago manual of style
A manual for writers of research papers, theses, and dissertations : Chicago style for students and researchers by 
Royalty Free Clip Art
Royalty free clip art, like the images available from Microsoft Office, do not need to be cited.
Online Citation Resources
Even with these online resources, you may need to consult the actual style manual to make certain you have formatted a citation correctly. You can consult a citation manual books at the Research Center Desk in the library, or use the Citation Guides listed to your left.
Citing Images
Images must be cited like all other resources. If you use an image you did not create (even if it's in the public domain), you must provide a citation. Whether you found an image for a paper or project through Google Images, a library database, or in a book, be sure to cite it properly. By citing your sources you avoid plagiarism and you give credit to the creator of the image, video, or text you are using. The citation should be accessible in the context of the image's use (within a PowerPoint presentation, on a web page, in a paper, etc.)
Image citations should include the following information at a minimum:
It is also useful to include date, culture, and rights information, if known.
Citation Styles for Images
There are different styles used to cite images. These include APA, MLA, and Chicago (Turabian is a condensed version of Chicago). Ask your professor which style to use and then check out these links to learn how to cite using the appropriate style.
Images in the Public Domain
No permission is needed to use works in the public domain because they are not protected by copyright. Most works enter the public domain when the law no longer considers them under copyright. View the Copyright and Fair Use tab for more information.
Is this Photograph in the Public Domain?
According to Wikimedia.org "this work is in the public domain in the United States, and those countries with a copyright term of life of the author plus 100 years or fewer." The Wikimedia Foundation adds, "faithful reproductions of two-dimensional public domain works of art are public domain, and that claims to the contrary represent an assault on the very concept of a public domain". However, the Prado Museum owns the painting and therefore, can theoretically claim rights to the painting. The photographer might also be able to claim rights to this reproduction of Velazquez's painting. The law is unclear in this area. What do you think?
Diego Velázquez [Public domain], via Wikimedia Commons
