There are a number of online sites to source photographs that are royalty free. Be sure to go through and comply with publishing and licensing policy clauses, generally listed on the source site. Here is the list of few online sources:
Flikr’s Creative Commons: Use the advanced search feature, it allows you to search for photos with a Creative Commons license for non-commercial use and images that you can modify, adapt, or build upon.
Wikimedia Commons : A collection of 36,305,116 freely usable media files, Wikimedia Commons allows you to search by topic, photographers, and location.
Google Images : After entering your query into Google Images, click the “Search tools” link at the top of the page. This will drop down a set of filters, including one for “Usage rights.”
Open Photo: It is a stock photography community and framework whose purpose is to allow photographers to share and protect their works through Creative Commons licensing. It requires you to credit the photographer.