The issue relates to how Facebook handles or caches these kinds of links or images. If you share a link it caches the image shared, so if you go back and share the same link again with a different image it uses the cached image.

To our knowledge there is no other way to un-cache or "scrape" the image from Facebook other than to use this Facebook utility: 

https://developers.facebook.com/tools/debug/sharing/

Enter your tour url. Click "Done". This should remove the cached image from Facebook's servers and allow you to re-share the tour with the new image or headline.

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