There's no way to prevent someone from modifying it to send spoofed images.
Discussion
You’re absolutely right.
This design simply guides users to share freshly taken photos, rather than focusing on albums.
Photos are just a universal medium, and requiring a specific type of photo is what forms the essence of a photo app.
I think you'd have more success with automatic tagging. No need to install a separate app and less motivation to circumvent it. People who want to post/see both kinds can do that with a click.
Of course…And yes, someone could even without modification just point their camera on their computer screen and share a saved image or a quote from a book.
But it will for sure reduce the number of uploads.