To do this you must have access to code of your blog post. In head section there are meta tags which define the way you share something on different sites. It is called OG tags and can also be done through some extensions
Here is simple example of meta tag changing.
<meta property=“og:image” content=“http://example.com/example.jpg” />
<meta property="og:image:width” content="180" />
<meta property="og:image:height” content="110" />
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