Around the world some people have made their creative work freely available for others to reuse.
It might be a simple reuse (with attribution to the author) or the person might allow others to use their work and alter it (remix).
If you want to use someone else's work, check if they allow it.
This page tells you about the main websites to find such information.
Google image search
Many people use Google image search as the 'go to' resource for looking for images they can reuse.
Do a search and then click on Tools
Click on Usage Rights and limit to any of the 'labeled for reuse' options.
A useful term to know about is Creative Commons
This is a licensing system which has created a body of work freely available for legal use, sharing, repurposing and remixing.
Through the use of CC licenses, people around the world have made their photos, videos, writing, music and other creative content available for anyone to use. Owners declare when uploading on to websites how they wish their work to be used.
Just remember to attribute (quote) that person if they have requested it.