Anyone who creates something, including art, photographs, books, movies, music, research articles, and so on, owns the copyright to it. So, if you want to reuse something that is copyrighted you have to ask permission from the creator, you might have to pay a fee to use the work, and you must credit the creator.
However, some creators allow people to use their work under a Creative Commons license. This is a way for creators to designate how they want their images, videos, etc. to be used. There are also other works that were created so long ago that they are now out of copyright and in the Public Domain. That means anyone can reuse them without permission.
This guide will help you to find images that are in the public domain or under a creative commons license. Remember to cite - or give "attribution" - for all the images or media you use in your research.