Just because someone posts an image or video online doesn't mean it's automatically in the public domain (in legal terms) and you have free reign to use it for personal, non-commercial use. The image is still copyrighted and owned by the author, so any use without the author's permission (and a credit to the author at minimum) could be considered illegal and the author has every right to demand your to cease using it.
I personally use this copyright on my site:
Basically says you use my photos else as long as you credit the photo correctly to me.
I have also prevented hotlinking (directly linking to an image on my server) in the past to prevent bandwidth abuse on old sites I use to run.