problem with Facebook and preview images

Hello everyone,
I have a problem with previews of the images on Facebook and I can not figure out where mistake.
So, I have a blog on Blogger and I would agree that when the home site to take a picture "generic", when I share the post to take the image in the post.
Without any intervention, if I share the home takes me a random image of a post (which I would expect to take at least the header, but nothing); for post recovers properly instead the placed image.
If I'm going to define in the destination code to tell it what image recover, it works but recovers that same image for all pages!

Is there a way out?

Thanks, I do not know what to do!

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