The Noun Project is an online catalog of free clip art that you can download and reuse for all kinds of projects. The catalog is divided into eleven categories including sports, safety, food, animals, and more. All of the images are either public domain images or Creative Commons licensed images. If the images are public domain you are free to reuse them without attribution without restriction. If the images are Creative Commons licensed, please note the attribution requirements.
Applications for Education
If your students need simple images for illustrations, diagrams, or multimedia projects, the Noun Project is a good resource for them to browse through.