I’m going to be a bit stern, here, because this keeps coming up in various forms. It’s really quite simple.
ONE SIMPLE RULE
Every piece of creative work is owned by someone. That is, the copyright belongs to somebody. To take someone else’s creative work and use it without permission is to break the law. It is theft. Just as much as if you had walked into a computer store, picked up a laptop, and walked out without paying for it. There is no difference in law. Taking a picture, a piece of music, a section of text, a piece of video without asking is stealing.
It really is that simple.
Now, on investigating who the copyright holder is, you might find that they have been dead for more than 70 years, so copyright has expired (see footnote), or that the work has been expressly released into the public domain on a Creative Commons licence or in some other way. In which case, you are free to go ahead. But you really should be sure.
Many copyright holders will not ask for money in exchange for granting permission. Many will ask only a small fee. But you should always always start from the assumption that everything you have not created yourself is owned by somebody, and you need to ask permission.
IT IS NOT FREE BECAUSE IT IS ON THE INTERNET.
The problem existed before the Internet, but the Internet has made it many many times worse. People assume – most often out of ignorance – that anything they find on the Internet is copyright-free. It isn’t. It might be, but you have to start from the idea that it isn’t free to use, and find out where to get permission.
There is a lot of confusion, here. For example, there’s a whole industry built around recycling CDs. People rip their CDs to their computers and digital players, and then sell the Compact Discs. This is theft. You can rip the CD for your own use, and you can sell the CD if you don’t want it any more. But selling the CD AND keeping the music is stealing from the copyright holder – the musicians – who have sold their work to one person (you), not two or more. You probably even know that to keep the CD and sell the digital copy would be piracy, but somehow it feels different the other way around.
POETRY AND PICTURES
More examples: poetry – writing of any sort – remains copyright until 70 years after the author’s death. It breaks the law to use a poem, or a song-lyric, or any other piece of writing without permission while it is in copyright. Similarly, pictures and photographs are the copyright of the photographer. There are places like pixabay.com where photographs are posted for you to use at little or no cost. But do not simply take a picture from an Internet search without finding out whether it is available for use.
(By the way, so-called “fair use” is part of US copyright law. It does not apply in the UK or Europe, and is anyway less straightforward than it might seem.)
Ultimately, this isn’t about complying with the law simply because it’s the law, but because the law is there to protect the livelihood of the writers, artists, musicians, photographers who make their living from selling their work.
That’s people like you and me. Even if it’s not your livelihood, your writing and photographs and songs are your copyright. You wouldn’t walk into a pottery and take a pot without paying. It’s exactly the same. It isn’t difficult. Thank you.
Generally, copyright in the EU and UK extends for 70 years after the creator's death. In the US it is more complicated. I am not a lawyer. It's your job to confirm whether copyright still applies.