Creative Commons (CC) Search is a great place to start looking for images with a Creative Commons license, which means that they are free to use as long as you attribute them properly. CC Search provides guidance on attribution.
Some common sources for images and photos online are Google Images, Flickr, and Wikimedia Commons. A lot of the content found in these resources is protected by copyright, meaning that you're breaking the law if you use it without permission from the creator. However, there are ways to make sure that you can legally use what you find!
For more information about reusing images from Wikimedia Commons, see their guide.