Once you have a set of images, click on Search Tools and select Usage Rights. This allows you to limit images that have been given a Creative Commons license allowing others to legally resuse their images. Teach your students to isolate these types of images and then site them appropriately. You name the license and give a link to it as well as a link to the image itself.
Here are eamples for citing Creative Commons images in PowerPoints and websites: taken from How to Attribute a CC Work:
Click on the camera icon in the image search bar to upload a photo or link to an image on the web. Google will then try and find similar images.
Use advanced Images search for image options such as colour and to select file size. If image is copyrighted, do not use in a permanent website or flyer without permission.
e.g. Escher; for coloration select black and white
“world war II” canada. Indicate the size of image that is preferred. Use right mouse button to save.