We have a number of dedicated websites containing thousands of images. Most of these images were taken by Dr.Martyn Overy.

Students and teachers are free to use these resources in their lessons and projects. They are for non-profit, educational use only.

Webucate.org Gallery    http://www.webucate.org/ourgallery/

GoScience.org Gallery   http://www.goscience.org.uk/gallery/

Mywebschool.org Gallery   http://www.mywebschool.org/postnuke/modules.php?op=modload&name=gallery&file=index

SupportScience.org Gallery   http://www.supportscience.org/cgi-bin/imagefolio/imageFolio.cgi

ScienceBlog.org  Gallery  http://www.scienceblog.org/community/modules.php?op=modload&name=gallery&file=index

Of course, you can try image searches on www.google.com  , and similar websites.   However, many images are subject to copyright restrictions.