My Facebook Thumbnail is not picking the image on my post.. why..?

Like I said, I also here. I have no answer yet.

Firstly, I suggest you install and use WordPress SEO plugin: http://wordpress.org/plugins/wordpress-seo/
With this plugin you will have an option to choose what image to share on Facebook in Post editor like the screenshot below:
 
If you are using another social plugin and images are still not replaced with the new image, please go to this page: https://developers.facebook.com/tools/debug/ and add your link to the post to reload images to share on Facebook.
Hope this helps!

i helped me. Thank you very much