The image and description that are shown when someone shares their unique link from the Viral Loops widgets (eg. popup or Sharing Widget) are retrieved from your page by Facebook.
Squarespace automatically specifies which image will be shared and that is determined on the type of content that is being shared: cover page, blog post, product page, etc.
You can find out more about that and about which image Squarespace promotes as "featured" by checking this article from their knowledge base: https://support.squarespace.com/hc/en-us/articles/205812778#toc-which-image-displays-in-social-posts
For further debugging, you can check out this article as well: https://support.squarespace.com/hc/en-us/articles/205814548-Troubleshooting-Facebook-sharing-issues#toc-facebook-is-sharing-the-wrong-image-or-content
Remember that at any moment you can check how Facebook "sees" your website by testing it on the Facebook Sharing Debugger. You get a response of which image will be shown in the Facebook posts, which description etc.
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