Photo by Nick Brunner on Unsplash
Until you publish your story, it lives only on your computer--either in the browser or in the desktop app. The Cookbook recommends two free hosting services--Borogove.io and Itch.io. Both allow you to choose whether your project is publicly available or restricted. Itch.io will allow you to upload a folder containing your Twine story and any associated media that you have saved on your computer.
The Internet contains millions of images, and it is usually easy to download a copy of an image for reuse in a presentation or some other project.
Keep in mind two things:
Many of the images online that can be freely reused have something called a Creative Commons license. Look for the CC logo as you are searching. There are different types of CC licenses, but all of them allow you to use a CC-licensed work for noncommercial purposes (like education), as long as you credit the person who created it.
Try some of the following sites to find high-quality, reusable images licensed with Creative Commons or similar open licenses. When reusing an image you found online, it is always recommended and often required that you attribute the image to the original creator/source.
You can search for reusable images in Google's image search by going to Tools and selecting Creative Commons under Usage Rights.
For additional music collections, search online for "royalty free music."