OK, thanks for the information. You must be using the option to upload your own photos. Behind the scenes, this uses a feature service to store your information, which has different security requirements.
If you are able to host your images somewhere else and use one of the other options to create your tour you will be able to use this technique.
This is mentioned in the note at the bottom of the blog article you're following.
Owen Evans
Lead Product Engineer | StoryMaps