You can find images on the web using an image search engine like Google Images or Picsearch, or by searching an image database such as Wikimedia Commons or Flickr.
You can also find images in databases such as Images Canada, the UNESCO Photobank, History of the World in Photographs, or the Earth Science World Image Bank.
Many libraries and museums have large digital image collections, including the Smithsonian Institute, the New York Public Library, the Library of Congress, the Virtual Museum of Canada.
See the individual Subject guides for more subject-specific image sources. There's also a Subject guide for images.