Recommended and mentioned products
Unsplash is a website with high-quality free HD images. It has a catalog of more than three hundred thousand striking images that are neatly organized with tags. Read more about Unsplash.
Unsplash has more artistic and stock-photo photography.
A site dedicated to sharing artwork & other assets for game development.
OpenGameArt.org has assets intended for use in video games, but many of these could plausibly be repurposed for a CYOA.
Internet Archive is a non-profit digital library offering free universal access to books, movies...
The Internet Archive has old book scans which sometimes have pretty good art that hasn't made it to an easily searchable area of Wikimedia Commons yet.