It's autogenerated by google-bot from your webpage. Mostly it will take highest resolution, largest size picture as serp thumbnail. You can change it on your page, and after next update, (in week or so) you can find new thumbnail.
It's a mix of schema markup and chance.....you can have a perfect schema markup and G bots can piss on it and display whatever they want.....you can control the thumbnail only for paid ads.
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