We don't detect other og:image tags. We do search (and remove) a lot of other og: tags to make sure there are duplicates because having multiple of some tags is a problem. If there are multiple og:image tags though, that's ok (in Facebook's eyes) as they can all be used.
We'd recommend using the JFBCSystem plugin *first* in the ordering as it *may* get our og:image tags higher up in the head section, so they are used first, but no guarantees on that.
I hope that helps answer your question, but if you need anything else, or have any feedback or suggestions for improvement, just let me know.
Finally, if you haven't already, please consider leaving a rating and review for JFBConnect, or our support, on the Joomla Extension Directory. It certainly isn't required, but is very appreciated:
extensions.joomla.org/extensions/social-...ook-integration/7215
Thanks,
Alex