I'm unsure what you mean here. For any Joomla content page, you can enable the Open Graph - Content plugin that comes with JFBConnect. Once enabled, JFBConnect will automatically set the og:image tag on the page.
If you have a specific page where the og:image tag isn't being set right, you can set the following tag in the article which will force the og:image to whatever link you want:
I hope that helps, but if you need anything else, please let explain what you're trying to do further and we'll help however we can.