When posting a link to my site in social media such as facebook and Twitter, and in SMS text messages, the preview image shown is for the template used to create the site, instead of the actual site.
I know it’s been over a year and you may have found your answer already, but I figured I would answer the question anyway to contribute to the forum.
What you are looking for is called the Open Graph Image. It can be found inside the page settings within the designer. The same place you go to update your Meta Descriptions, etc.