The OG:Image plugin adds an og:image (Open Graph) meta tag to com_content pages based on the article image or a configurable default image..
The article-specific image must be stored in the article under “Images and Links” or, in the case of a category, under “Options.” It takes precedence over the default image specified in the plugin settings. The default image is displayed if no article-specific image has been specified..
Setting a post-specific image
Pasting a URL on Facebook - OG:Image is taken into account"
