All the 404 error page images that are available on the 404 Chef template page are free and licensed under Creative Commons 0 (CC0) license. Feel free to use them in any of your projects, commercial or not. No copyright notice necessary.
Bon Appétit! / 404 Chef /

The handmade 404 error page images, however, are exclusive to the person or company that bought them. The 404 Chef can cook vegetarian 404s as well!