1) Look at the first row of icons on your dashboard. 2) Click the one that has "Photo" written under the camera icon. 3) Add "#John Green" or something relevant as one of the tags. 4) Click "Publish" and hope he sees it.
Or you could click the "You can ask me questions only if you agree not to get mad if I don't answer." button that John has, give him the link, and hope he sees it.