If you're using Weebly, start by viewing the published site and right-clicking the image to Copy Image Address. Then in Weebly, go to Edit Site, Pages, click the page you wish to use, SEO Settings, under Header Code enter the code from Shef's answer:
<meta property="og:image" content="/uploads/..." />
just replacing /uploads/... with the copied image address. Click Publish to apply the change.
You can skip the part of Shef's answer about namespace, because that's already set by default in Weebly.