From your dashboard, click on Sites then select Funnels or Websites depending on where the page is located.
Choose the funnel or website that contains the specific page, then click Edit .
Once the editor opens, click on the SEO Meta Data icon on the upper left side of the page.
On the left side panel, click on Social Image and upload the image you want to appear when the link is shared.
Once everything is set, click Update SEO Meta, Save and then Publish to push your changes live.