Here are a few links to search engines that search for royalty-free images and images under Creative Commons licences.
Note: Before using any image, make sure the use you wish to make is allowed by consulting the conditions of use that apply to the selected image.
Whether you have obtained permission to use an image or have used a royalty-free image, your sources should be cited.
• Wikimedia Commons
• CC Search
Google Images: In the advanced search, filter results by usage rights:
APA and IEEE Citation Guide