Facebook crawls sites for the first image it sees when a post is shared to it. You can set this image for all posts shared to Facebook for School or District sites.
This default image must be called ogImage.png and has to be at least 300 x 300 pixels. To make this image avaiable, upload it to the Style Library in the Images folder.
To upload the ogImage.png file:
It may take some time for Facebook to crawl through your site to automatically populate the image. If a school site does not have an ogImage.png file, Facebook will default to the District's logo.