You forgot an essential part of the issue, the most important part I would say,
its called the "intent to deceive". For example, even if you get a pic from say pixabay, and then post it in the photography tag pretending its yours, thats still plagiarism, plagiarism and copyright, you must understand, are separate even though partially overlapping concepts. In the opposite example, if you take any picture but clearly denote that the image was not taken by you, thats not plagiarism.
I hope this adds a bit of nuance to your article.