Most of the photos you find in the internet are not free to use. Be careful with the author's rights!
Here you can find photos you can use freely:
https://pixabay.com/es/
http://recursostic.educacion.es/bancoimagenes/web/
https://search.creativecommons.org/
https://commons.wikimedia.org/wiki/
https://www.flickr.com/search/advanced/
If you have any doubt, ask your teacher.