@CheekyRadio Just to add to @Adam's reference post, you can upload your desired OG image to your Weebly file store. To do this, in the Site Editor, click Theme > Edit HTML/CSS (button at bottom of left-hand sidebar). In the left-hand sidebar, scroll down to the "Assets" section and click the "+" to the immediate right of the Assets label. Select "Upload File(s)" and then browse to the location on your computer where your desired image is.
Now, let's say that you named your image "fb_og_image.jpg" and your site's domain is "mysite.com". Per Adam's post, in the Site Editor, click Settings > SEO then scroll down the "Header Code" input box. Add the following OG image meta tag:
<meta property="og:image" content="https://mysite.com/files/theme/fb_og_image.jpg" />
Click the green Save button at the bottom of the Settings page and then re-publish your site. You can use the Facebook Sharing Debugger to preview how your site link will appear in a Facebook post: https://developers.facebook.com/tools/debug/. Or you can just create a new Facebook post and add your site's url within the post to see how it previews (you can see this before you have to actually save the post). If you want to implement different OG images on different site pages, as Adam mentions, click on Pages in the Site Editor then click your particular page, click SEO Settings then add your OG image meta tag to the "Header Code" input box there (in this case, you'll want to upload multiple images to your file store and then reference them on each of the desired pages by modifying the filename in the meta tag).
