When you share a post of your drupal site on facebook, it will automatically detect the images of your node for you to choose.

But sometimes, it does not detect any images, or the detected images are not what you want. 

You can always upload the main image manually. That's fine, because you are the owner of the site, you know which ones to choose. However, if your readers share that piece of content and it has an ugly image, it does not look attracting at all.