Each of these sites has free clip art that can be used on educational web sites.  Please be sure to follow the rules of the site for providing copyright information whenever you make web pages.

Discovery School's Clip Art Gallery

http://school.discovery.com/clipart/

Teachnet.com

http://www.teachnet.com/how-to/clipart/

Educational Clip Art

http://www.teacherfiles.com/clip_art.htm

School Clip Art

http://www.datasync.com/~teachers/school_clipart.html

The Teacher's Guide Free Educational Clip Art Page

http://www.theteachersguide.com/Freebies.html

Clipart for Students and Teachers

http://www.stemnet.nf.ca/CITE/clipart.htm

The Clip Art Locator

http://www.thermix.com/naje/clip_art.htm

Dreamcatcher Graphics

Includes complete web page sets

http://www.dreamcatcher-graphics.com/ensembles.htm

Animations Planets

http://www.geocities.com/SouthBeach/2177/Anim2.html

Clip Art, Sounds and Fonts Galore

Includes clip art search engines

http://library.hilton.kzn.school.za/Mainpgs/clipart&.htm

Lesson Planet

Teacher Web Guide

http://lessonplanet.teacherwebtools.com/clipartdirectory.html

Barry's Clip Art

http://www.barrysclipart.com/index.html

Clipart.com

http://www.clipart.com/ 

Mousepads Kids by Vikimouse

http://www.vikimouse.com/ 

Clipart Connection

http://www.clipartconnection.com/ 

Clipart by The Wizard of Draws

http://www.wizardofdraws.com/

Phyllis's Backgrounds

http://www.eskimo.com/~physmith/Backgrounds.html