1. Just because it's on the internet, doesn't mean you can use it. Photos and other created images belong to their creator, and only they have the right to reproduce, change, or use the image in another product. If the image doesn't have a Creative Commons licence or otherwise specifically indicate that it is free to use or public domain, you can assume that it is copyrighted and you shouldn't use it without permission, even if you add a citation to indicate where you got the image.
2. To find free to use images in Google images, go to "Settings" then "Advanced Search," enter your search term at the top, then scroll down to the bottom and change the usage rights to "free to use or share."
3. There are several sites you can use to find free, or "fair use" images. You must still give a citation, preferably directly under the image. The attribution should include the Title of the work, the Author, Source and Licence. Click here for examples of how to write an attribution. Try the links below.
example of a Creative Commons Licence