All resources used in research must be cited, including Open Source/Open Access sources. Remember not all Open Access Images are licensed the same way. Always double-check the usage rights for any image that you plan on using, no matter the nature of your use. If the usage rights are not on the main page of the site, look for a How can I use it section of each description. If you still have questions about using an open image ask your professor or the librarian.
Keep in mind these possible related limitations on image use:
Courtesy Drexel University Libraries
Open Access does not mean free for the taking. It means the source is available to be used responsibly. All sources used in research must be given proper credit.