Respect other people's material. It is called the copyright or copyright to materials created by other people and that are not allowed to be disseminated or used.
We can choose photographs or other free graphic resources from the following websites:
FlickrCC.
Banco de imágenes del CNICE.
Mediateca de EducaMadrid.
Open Clip Art Gallery.
Openphoto.
Free Digital Photos.
Pixabay
Pexels
Stockpic
Freepik
The Noun Project
Thinglink