In general in your course assignments you can, under fair dealing for purposes of research, private study and education, use one entire image from a compilation of images (e.g.
Most images you find on the Internet, in books and elsewhere are protected by copyright.
The act of creating something automatically gives it copyright protection.
Examples are materials posted to the Web under a Creative Commons license, or materials that are out of copyright and now in the public domain.
You can search for such material using the Creative Commons search engine.