If you're using standard Joomla content, you should just need to enable the Open Graph - Content plugin. That will try to automatically detect and set the image for each article.
On each individual article, if you want to set a different image for some reason, you can add the following tag to the article:
{SCOpenGraph image=http://yoursite.com/link-to-image.jpg}I hope that helps, but if not, let us know.
Thanks,
Alex
OpenGraph image dynamically added from articles
