These sites offer free graphics, as well as ideas and activities for students who benefit from visual supports:

http://www.pics4learning.com - categorized lists of images, free for educational use.

http://hcmc.uvic.ca/clipart - over 3000 pictures targeted at teaching vocabulary in a variety of languages. 

http://www.symbolworld.org - pictures, activities, stories and more.

http://dotolearn.com - print resources, songs, games and visual supports for students with special needs.

http://www.tinsnips.org - resouces for students who have autism spectrum disorders and other special needs.

http://www.through-the-maze.org.uk/pages/Symbols - symbols arranged by category.