Images found online are protected by copyright. Many of the images are posted without permission from the copyright owners. By using this guide, you can select from websites with clear terms and conditions allowing reuse. NOTE: Any person can take someone else's image and put a creative commons licence on it. Use your common sense as to whether it is likely that commercial and valuable images will be available for free.
You can limit a Google image search to Creative Commons images only by selecting "Tools" then "Usage Rights" and chosing one of four options 'Labeled for reuse', 'Labeled for reuse with modification', 'Labeled for non-commercial reuse' and 'Labeled for non-commercial reuse with modification.' 'Non-commercial reuse' would apply to classes at Monash and 'with modification' if you plan on altering the image.