Facebook can automatically pick up an image from your articles when they are shared by you or anyone else. Usually it picks the featured image of an article. However, sometimes it may pick a random image from the article that may not be suitable.
If you are using Yoast SEO plugin, then you can select a Facebook thumbnail image in Yoast SEO metabox on the post edit screen.
You can also select a Facebook thumbnail image for your homepage by visiting SEO » Social page.