When it comes to trying to find an image, subscription-based image databases are a great resource. The University pays fees to a number of organisations who provide images for educational re-use. You can freely use images from these sites without worrying that you are not complying with copyright law. They also provide access to thousands of images which you might not be able to find elsewhere, and offer far more comprehensive searching than a site like Google Images. Bridgeman Education, ArtStor, and ArtUK are great examples of sites which University staff and students are free to use in presentations.