How To Get Facebook To Change Your Post Link Image
When you change the image on your blog or web page you need to let Facebook know so they show the new image. They have this nifty tool called the Sharing Debugger, here's a quick video to show you how to use it.
Why should you do this? For congruency across the web. If you're sharing something on Facebook that looks one way and the web page looks completely different chances are higher that the user will click off because it doesn't look the same to them.
It's the little things that make a big difference.