When you share a post on Facebook, or when Jetpack Publicize publishes a post to your Facebook page, Facebook crawls the page and looks for Open Graph meta tags in the head to build a complete post preview (with an image, title, description, ...).
Jetpack automatically creates these Open Graph Meta tags for you, and looking at your site, the tags appear to be there, including an image tag. Here is a screenshot of the tags added by Jetpack to your latest post:
http://i.wpne.ws/YPDF
As you can see, it includes a link to your post's featured image:
http://sehat-tefl.com/wp-content/uploads/2014/11/1.jpg
Facebook sees that image when crawling your post, as you can see by entering the post's URLs in Facebook Debugger:
https://developers.facebook.com/tools/debug/og/object?q=http%3A%2F%2Fsehat-tefl.com%2Farchives%2F44
http://i.wpne.ws/YOci
Now if the image doesn't appear on your Facebook Page, I'm afraid there is not much you and I can do about it. If it keeps happening, I'd suggest contacting Facebook to let them know about the bug:
https://developers.facebook.com/support/