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?

This is something that you can't do with the metainfo dashboard feature.

It's intended to be used to specify an image link by link to have a fined tune control, so this is outside the scope.

You should search a third party plugin that allows to do that, but really don't know if it exists. It doesn't make so much sense having the same image for all posts, for all articles, etc It would be ideal if Kunena, Easyblog, etc would have a built-in feature to get automatically the contextual social share image taking it from the post contents.

Oki. Thanks.

a fallback meta image could be a nice feature for the source configuration.