2. Creative Commons licences

The safest way of identifying and using images that are free of charge is to use a system known as Creative Commons Licensing. Creative Commons (CC) is an American non-profit organization devoted to expanding the range of creative works available for others to build upon legally and to share. For a brief introduction to Creative Commons this 3 minute  video (free to share on Vimeo) gives you a basic introduction. For more information, have a look at its website here.

The Creative Commons website is an excellent place to start if you are searching for a Creative Commons image. You will find the search pages here. Just enter the term you are looking for and check or uncheck the relevant boxes according to the kind of resource you are looking for and where you would like to find it.

There are seven levels of CC licences, but they are different combinations of answers to three questions

1) Can the material be used commercially (education is not "commercial")?

2) Can people adapt your material (usually a good thing to allow)?

3) If they do adapt it do you require that they also share their material freely (sharealike)?

Creative Commons licence Rules
CC0
"Public Domain"
No rights reserved. Legally people do not have to cite the source, but it is good practice to do so
CC BY This license lets others distribute, remix, tweak, and build upon your work, even commercially, as long as they credit you for the original creation.
CC BY-SA This license lets others remix, tweak, and build upon your work even for commercial purposes, as long as they credit you and license their new creations under the identical terms.
CC BY-ND This license lets others reuse the work for any purpose, including commercially; however, it cannot be shared with others in adapted form, and credit must be provided to you.
CC BY-NC This license lets others remix, tweak, and build upon your work non-commercially, and although their new works must also acknowledge you and be non-commercial, they don’t have to license their derivative works on the same terms.
CC BY-NC-SA This license lets others remix, tweak, and build upon your work non-commercially, as long as they credit you and license their new creations under the identical terms.
CC BY-NC-ND

This license is the most restrictive of our six main licenses, only allowing others to download your works and share them with others as long as they credit you, but they can’t change them in any way or use them commercially.

I really like the picture below by Winslow Homer. I know I can reproduce it here because it is in the public domain - CC0, like 3 million other items from the Smithsonian Institute.

The Houses of Parliament by Winslow Homer