My site has four sections:
Forum (Kunena)
Blog (EasyBlog)
Community (Jomsocial)
Info (Joomla articles)

I would like to have one Open Graph image for each section. So if the post/page doesn't have an image, my image will kick in.
But, the forum has 70.000+ posts and the blog 7.600+. It's impossible for me to edit "Image for social share" for each and every link. And I can't keep doing this for every new post there is.

How do I do to specify one "Image for social share" each for the three components and one for everything else?