Just to add,
http://www.freepik.com/
http://www.clipproject.info/
https://www.pexels.com/public-domain-images/
http://www.arthursclipart.org/
http://freestock.ca/index.php
http://www.pics4learning.com/index.php
http://www.surfnetkids.com/resources/free-clip-art/ I have not checked out all of the suggested sites listed at surfnetkids, but I recognize 1 of them: http://classroomclipart.com/
http://www.school-clip-art.com/ was also recommended by surfnetkids, but I did not know of it before now.