When you share a post on any major social media channel (by copy/pasting the address of a post) it's much more fun if the link can display an image. Basically this is the first image found on the page. Since I am mostly doing this don't be surprised if you see a seemingly unnecessary image added by the administrator to otherwise bare post with text only. This doesn't mean the post is not good enough to exist without an image but that I have shared it on Facebook or someplace else.