Images that have been labelled as Creative Commons have been shared freely by the original creator.
You can narrow down your Google images search results to only include images that are free to use under creative commons.
From the results screen select Tools > Usage Rights > Creative Commons licenses
In Australia any creative commons image you use needs to be attributed. This can simply look like the name of the creator and a link back to where you found the picture. An attribution should be placed close to the image.
Attribution example
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There are many websites where you can find free images. It's best practise to include an attribution close to every image used.
For a longer list of free images, graphics and creative commons content see the Need Free Images page of the copyright library guide.