Here's another great free resource from Discovery Education - free clip art. Discovery Education has a great Clip Art gallery in their Classroom Resources section.
The Clip Art Gallery has hundreds of original clip art, including animations, sorted into categories such as science, food, music, math, letters and numbers and much more.
All of the images are ready to use and many are available in black & white or color. You simply go to the image you want and then right-click to save the image.
If you are looking for free images for your project, blog, presentation, or website, check it out.