When you click on a thumbnail in 'Google Image Search' it says "Images may be subject to copyright."
It is important to understood that the images don’t come from the search engine and that they are only identified by it. You don’t get your picture of a black swan in the sunset from, for instance, ‘Google’ - it comes from the website that hosts it; so you have to learn that you are reaching through ‘Google’ back to the source website to get the picture.
Australian Black Swan. NEN Gallery, by Paul Wright. NEN Gallery images can be "used freely for educational use ... this includes the editing and repurposing of these resources for use in education. It explicitly forbids any commercial use. It is expected that wherever possible the credit will be given to the resource owner"
‘Google’ Advanced Image Search provides options to find images which can legitimately be re-used or re-mixed. The categories are:
In 'Google Image' the selected image is shown over it's source so that you see what the source was, perhaps who owns the copyright or at lest what type of copyright it is and get more information about the image - when it was taken; what's in it; the context it was used in.
Advanced search also applies to other serach engines - it's not just a Google thing.
Going back to the source is not only a good way of becoming more certain about the copyrights that might apply to an image but usually provides further information about it, where it comes from, what exactly is shown in it and the context in which it has been published. There is also more chance of getting a good quality image.
Image Searching Skills: an interactive tutorial from JISC’s VTS to improve your image searching skills. http://www.vts.intute.ac.uk/tutorial/imagesearching/