Use the 'intro image' as open graph default image

First, enable the 'Open Graph - Content' plugin since it looks like you're probably using Joomla articles. Then go into JFBConnect > Open Graph > Objects and create a new object. Then make sure to set the category appropriately and select Image Type to be 'Article Intro Image'. When JFBConnect goes to add the Open Graph tags to the page, it will look at the object set up and set the og:tag according to this object.

Here's a screenshot of an example Joomla Article object in JFBConnect.

Let me know if you continue to have problems after trying this suggestion.

Thanks,
-Melissa