You shouldn't need to edit 20,000 pages manually
Do the following:
* Go to the Plugin Manager and enable the Open Graph - Content plugin.
* Go to the JFBConnect -> Open Graph area
* Create a new Object for Joomla Content - Articles
* Set the type to 'article' and check "Built-in Facebook Object"
* On the right side, set the category to 'Root' or 'Top Level' so that this object applies to all articles on the site
* Set the Image to Article Intro Image or Article Full Image (whichever works for your site)
I didn't detail each parameter above, but hopefully that gets you going. If not, or you have any questions, just let us know.
Thanks,
Alex