I think that it's still the user's job to make sure that they are skeptical of the provenance of any photos that claim to be from, say, the NY Times, that are not viewed in the NYT's viewer (if they were using your system). And then, they should still trust the image only as far as they trust the NYT. But if they're viewing the image the "right" way they can generally believe it's what the NYT intended to put out.

And perhaps, over time, user behavior would adapt to fit that method of media usage, and it would be commonplace.

I am skeptical that that "over time" will come to pass. And I think that users will not be apply appropriate skepticism or verification to images that fit their presuppositions. And I think malicious players (like some mentioned in the article) will attempt to build and propagate user behavior that goes around this system (sharing media on platforms that don't use the client, for instance).

And I guess making that broad set of problems harder or impossible is really what I'd like to solve. I can see how your startup makes good behavior possible, and I guess that's a good first step and good business case.