Discovery Education clip art for various subjects and fun.
Classroom Clipart has an extensive gallery of clip art for subjects there is also clipart for each state if you are doing a state study.
Free Teacher Clip Art great for newsletters also contains currency very realistic and place value.
Karen's Whimsy has both color and black and white images with a huge selections of topics to chose from.
U.S. History Images the most extensive I have seen Native American images by tribe and so much more.