Start by downloading YouTube videos or by having an able-bodied person shoot video and upload it to your computer. Then edit them in iMovie and upload the edited video to Keynote. All of this software is readily available and usable with a headtracker.
Still photos are even easier. Just download them from the Internet or a camera and upload them to Photos for editing. When you're happy, upload them to Keynote. Once again, all of this software is readily available and usable with a headtracker.