Images found online are subject to copyright law just like any other creative work. In other words, just because an image is freely accessible on the Internet does not mean it is legally available to use in an assignment or project.
This guide provides:
To appropriately credit an image on a website you have created, you can follow this format:
"Image Title" by Creator's Name is licensed for reuse under CC License Type.
"Monarch Butterfly" by Peter Miller is licensed for reuse under CC BY-NC-ND 2.0.