You're welcome @supernovastaffy. You can use CC0 or public domain images. I think the topic on using online images has to go on a different post as I have only touched on it a bit. I focused more about plagiarism in general.
You can check out sites such as Pixabay or Pexels, these are just the popular ones. These are free to use and just cite the source under the image. You can also check on images if they require attribution. If it's not clearly stated and you're not sure, better search for something else.