Images on the web
There are sites that provide copyright cleared images that you can use, for example http://search.creativecommons.org/
Here you can search for images that have been created using Creative Commons licenses. That means you can copy and/or re-use those images.
Other sites that provide images you can use in teaching
Wellcome images http://wellcomeimages.org/ Images from the Wellcome Collection, covering medicine, social history and contemporary healthcare.
VADS Online resource for visual arts http://www.vads.ac.uk/collections/index.html
JISC Media Hub http://jiscmediahub.ac.uk/
Courtauld Institute of Art http://www.artandarchitecture.org.uk/about/courtauld.html