Just to be clear, Facebook's 'Post to Facebook' dialogue for sharing links is using Facebook's own database of information about links.
Sorry if I didn't make this clear before, but the way it works is that because Facebook has so many active users and many of them will share the same URL, Facebook only fetches a thumbnail for a page's URL the first time that someone puts that link in Facebook's database.
In your case, it fetched it the first time that you shared the synth.
Then when you saw the thumbnail, edited the synth to have a more desirable thumbnail, and reshared the link, Facebook just pulled up the thumbnail that it already stored for that particular URL.
The only way to get Facebook to fetch an updated thumbnail for that exact link is to open that debug page, erase Facebook's information about that link (previously fetched title, thumbnail, text summary, etc.), and then share that link again, which will allow Facebook to get an updated thumbnail etc.