Here are a few links to search engines that search for royalty-free images and images under Creative Commons licences.
Note: The links to these sites are for information purposes only. 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.
Google Images: In the Google Images advanced search, filter results by "usage rights", as illustrated below.