We did a posting recently on the thorny issue of when can one use photos garnered from the internet to spice up blog postings or presentations. You need to understand the concept of creative commons and once you have then here are 6 sites that we use to get free photos and artwork – admittedly with some conditions on some of them.
http://www.freedigitalphotos.net (you have to give acknowledgement of the site/photographer)
http://www.flickr.com/search/advanced (make sure to tick the creative commons box in advanced search)
Hunting them down takes a lot longer than just reproducing something from Google or Bing images but if you do you are much more clearly protected.
Happy hunting. The one at the top here is from www.sxc.hu