Images can add interest and substance to your work; however, many images have copyright or attribution restrictions. Please check for restrictions before using an image.
For a brief introduction of copyright, fair use, and related issues: "Copyright Overview," by NOLO, with Stanford University Libraries.
It is recommend that you use works licensed under a Creative Commons license if you want to avoid violating copyright.
Another good source: Finding Public Domain and Creative Commons Media, a guide from Harvard University Law School Library.
One example of a source on Harvard's "Finding Public Domain..." guide is The Public Domain Review.
You may also want to check the list of Public Domain Image Resources in Wikipedia.
This site was not written by an attorney. The information provided here is not legal advice. Links are not endorsements of any product.