Two days ago I shared with you a link to source of free photos called Veezzle. Today, I'd like to call your attention to Open Clip Art Library. Open Clip Art Library is a gallery of more than 31,000 clip art images that have been released into the public domain. All of the clip art in the Open Clip Art Library can be downloaded and reused for free.
Applications for Education
If you or your students are looking for some clip art that is different than the standard images found in common presentation tools, the Open Clip Art Library is a good place to find it.