These days, teachers are expected to reference and cite every image, especially if used in an assessment task.
However, photos on the internet are copyright the photographer - unless otherwise marked. Probably.
To use an image legally, you either need to
- get permission from the photographer or author (if "All Rights Reserved"), or
- find an image that is licensed under "Creative Commons"(you must credit the author), or
- find an image that is in the Public Domain (totally free, no need to cite source - usually very old works)
- find images from a place like Morgue File, where photographers have donated photos from previous projects (copyright-free!!).
You can search several places for creative commons licensed images: for example:
