Google simply displays results it finds on the web. It could be images from a site that provides images "free to use", but it will also display copyrighted images. You will have to follow the link to the site hosting the image, and determine if it is copyrighted or not.
There is also a feature in the Google Advanced Search that will return only results that are "Creative Commons/Free to use", but you should still double check. "But Google Said So" will not be a valid defense for copyright infringement.