If I understand you correctly, you want Facebook NOT show images they have scraped from your site (because they might be out of context) and want to know how to instruct them or simply stop them from doing it.
You can also just create a dynamic rule that will redirect any attempt to pull certain files back to a single file, sort of a "Neener Neener" kind of result to the requester. Either way, this generator might be useful. Especially if you always want them to get the same image like your logo no matter what "they" grab automatically.