Images are intellectual property, and as with any form of intellectual property - like books, journal articles, etc - it's important to be an ethical information user. Just as you always cite your sources for ideas that are not your own, you should always cite the images you're using in your assignments, unless you happen to be the person who created them.
The slideshow below gives examples on copyright and fair use when it comes to images, and below that you can see examples of rights statements. You should always look for rights statements when you're using images, as they'll tell you how you can and can't use the image.
Finding & Using Images for Your Work
Rights Statements Examples
Take a look at the rights statements listed below, grabbed from various websites and image repositories. Try to figure out which one(s) would be okay to use for a classroom assignment.