This page contains a gallery of free, public domain header images. Feel free to use these, modify these, whatever you want.
To download an image, first click it, then right click on the image displayed in the popup, and save the image to your computer. You will get the full 940×198 pixel image.
After you’ve downloaded a header to your own computer, you can then use the Appearance->Header menu from your site’s control panel to upload and use the header on your site.
Many the header images on this page were found at http://www.public-domain-photos.com/. That site has many more public domain photos suitable for making a site header.
