The simplest way to locate an image is using Google's Image search. If you need to use an image, however, you may find those images to be of low quality and/or you may not be able to use the image due to copyright restrictions. The following sources contain many high-quality images; in most cases they may be reproduced for academic purposes.
ARTstor (off-campus use for UGA affiliates only): Over one million images of art, architecture, fashion, folklife, decorative arts, landscape design, maps, manuscripts. Searching and viewing images does not require a login, but to download images you'll need to create a free account.
OAIster: A central catalog of digital collections from libraries, museums, and archives worldwide. Use the "Format" option on the search screen to limit to images and other visual materials.
Digital Public Library of America: DPLA has a growing collection of digitized images, mostly historical photographs, from libraries across the U.S.