Most images on the internet are copyrighted and so should not be re-used. They are covered by particular types of licence and many have to be paid for (royalties). Those that you can use are often covered by creative commons licences which ask you to credit the owner of the image in a variety of different ways.
Two popular sources of free/attributable images are Wikimedia Commons and Pixabay
Over one million royalty free photos, illustrations and vector graphics – and thousands of free video clips. All images and footage on Pixabay are released under Creative Commons CC0: Free for commercial use, no attribution required.
Pixabay is the most valuable free alternative to commercial stock photo sites