Copyright Laws for Using Pictures on Internet / Public Domain?

I was wondering if there are any kind of laws restricting the use of pictures in the public domain for your own use? Mainly, my question is in regard to pictures I find online on public websites of things and/or people. My question is for example, I find a funny picture of a person and used it on a poster or... show more I was wondering if there are any kind of laws restricting the use of pictures in the public domain for your own use?

Mainly, my question is in regard to pictures I find online on public websites of things and/or people. My question is for example, I find a funny picture of a person and used it on a poster or something and sold it for a profit. Could I be sued for doing this?

If someone could explain to me when I can/can't use pictures I find on the internet for profitable purposes. From what I understand, anything found on the internet is in the public domain?

Thanks in advanced for the input!
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