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.