Images can say quite a bit and can really help to deliver the message on a presentation. It is important to realize that not every image that you find on the web is freely available. Most images are protected under copyright, and the use of these images without permission is illegal. This includes images that you may find on Google Images, Flikr, tumblr., etc.
There are dozens of websites dedicated to freely accessible images and other media that are perfectly legal. Below, I have included a short list, and each site allows browsing and searching for images. Often times, images from these sites are at a higher resolution than ones that you may copy-paste from online.
Life Magazine Images Archive from Google
Library of Congress Digital Image Archive
Wikipedia Here you will find a full list of other public domain image repositories.