We’ve got a great resource for you! ibiblio hosts WP Clipart, a collection of over 20,000 public domain images. The collection is searchable directly from the website, and users can also download the complete package that includes an image editor application.
The images can be saved as PNGs and JPEGs, and are available for use without attribtion (though attribution is encouraged).
This is a great asset that we hope you will find useful for web publishing, presentations, and reports.