I just recently installed a new module on this website, the metadata module, which once again brings control into our hands, now when articles, projects, and blog posts are shared on social media the pictures we've defined will be shown, rather than no image, or worse, some image that Facebook just pulls out of thin air.
     For example this post, when shared on social media will have the picture in the upper corner here shown, or that will be an option anyway, all our posts don't necessarily have an image in them so we defined a few default pictures, two versions of our logo, we also made the authors profile picture available to be used as the picture. I have no idea how Facebook or Google+ decides which image to use when there are multiple, but a little more experimentation should reveal that to me.

     NOTE: this only applies to volunteers. If you are going to use an image in your post that you want to show when shared please add the image to the image_field of the "add content" page, and then copy the image link and use it in the editor. If you add an image through the editor it won't be detected and won't be available, as it won't be in the image_field.