If you create a site that you initially test by publishing to Webflow's platform, then export the code to host on your own server or host, any OG images that you have added will still be linked to the Webflow publishing point. If you were to then unpublish the Webflow version of the site, those links will now be broken and must be manually changed to point the image URL's to the correct images on your server/host.
Here's what the OG image tag looks like now in the downloaded version of the code export -
<meta content="https://uploads-ssl.webflow.com/.../605fb59a8c5edcdbf7905_yourOGimage.png" property="og:image">
So, if you unpublish the Webflow version of the site, the image that the link points to no longer exists. You have to change the tag to -