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.