When you upload the photo, at risk of great confusion they could essentially watermark the photo or add a banner showing the location and perhaps some of the other key details, like camera model, right on the photo so it would at least get across to the user that there is an association between these two things that needs to be disabled.

To dismiss the banner you'd have to click a dismiss button which would ask you to confirm that you want to get rid of the location data completely. Then there would be a tiny little button that says “hide this location inside the photo, where I can't see it easily, but everyone totally could”. (But less stupid.)

It would be terrible because there would be huge support threads on why it's trying to share an image with an overlay, but it would get it across. Would be a different failure mode for user privacy than what you would have with a text prompt or an interstitial or whatever.